WO2013153397A1

WO2013153397A1 - Improved blood glucose testing with hematocrit correction

Info

Publication number: WO2013153397A1
Application number: PCT/GB2013/050947
Authority: WO
Inventors: Phillip J Ainger; Matthew Robert Bryan
Original assignee: Phillip J Ainger
Priority date: 2012-04-13
Filing date: 2013-04-12
Publication date: 2013-10-17
Also published as: GB201709777D0; GB2501128A; GB2548295A; GB2501128B; GB2548295B; GB201206589D0

Abstract

A sampling plate (30) is provided for use in performing electrical measurements using a measurement device (60) on a liquid sample containing blood comprisinga first sample zone for receiving a first liquid sample. A first pair of drive electrodes (31, 32) is provided with separate respective electrode terminals spaced by a spacing (33) for receiving a first liquid sample within the first sample zone for use in driving an electrical current through the first sample. At least onefurther sample zone is providedfor receiving a further suchliquid sample and containing a reagent to react with free glucose in the liquid sample. A further pair of drive electrodes (37, 38) are provided with separate respective electrode terminals spaced by a second spacing (34) for receiving a further liquid sample within the further sample zone for use in driving an electrical current through the further sample. The first spacing is less than the second spacing.

Description

IMPROVED BLOOD GLUCOSE TESTING WITH HEMATOCRIT CORRECTION

The present invention relates to a sample measurement system. In particular, the present invention relates to the measurement of properties of liquid samples of (or containing) blood. In particular the invention relates to a sample measurement system for measuring certain selected properties of a liquid substrate, such as the glucose levels in a blood sample. The invention also relates to a sampling plate, a measurement device, a data carrier containing software to operate the measurement device.

There is a widespread need for improving the accuracy of sample measurement systems such as those enabling e.g. a diabetes sufferer to know their blood sugar levels - i.e. the concentration of glucose in their blood.

Existing sample measurement systems use a measurement device which receives and takes measurement readings from a sampling plate spotted with a blood sample from a user. The sampling plate is often rectangular and is end-loaded with the blood sample. The blood sample, once loaded, is usually drawn into a sample zone having a number of sampling zones from which measurements are taken by the system.

Each sampling zone typically has its own particular contents. For example, the first sampling zone may have glucose oxidase deposit within it, a second deposit comprising a mixture of glucose oxidase and a predetermined amount of glucose, while a third sampling zone may contain no deposit. As the blood sample is drawn over all three sampling zones, chemical reactions occur with the deposits in each sampling zone, resulting in discrete electrolytes. Each sampling zone bridges a corresponding pair of electrodes. A potential difference is established across each sampling zone, via the electrodes, when the sampling plate is inserted into an operating measurement device. Electric current readings for each sampling zone then provide measurements necessary to assess the blood sugar (glucose) levels. For instance, the first sampling zone may give the primary measurement, whereas the second sampling zone may provide a degree of calibration since a known quantity of glucose was already present there. The third zone may give a final check by accounting for the non- glucose contribution to the measurements in the first and second sampling zones.

However, in spite of these calibrations and final checks, error margins in such blood glucose readings are still high. The present invention aims to address this. Blood plasma is the liquid component of blood in which the blood cells in whole blood are normally suspended. Blood plasma typically constitutes about 55% of the total volume of the blood. It is the extracellular fluid part of blood and is mostly water but contains dissolved glucose and other contents.

The volume percentage of red blood cells in blood is known as the haematocrit (HCT). Other terms for this are the packed cell volume (PCV) or erythrocyte volume fraction (EVF). Haematocrit is normally about 45% for men and 40% for women. The haematocrit is typically calculated by multiplying the red blood cell count in a blood sample by the average cell volume, then dividing the result by the whole blood sample volume. At its most general, the invention in one aspect is a system to measure haematocrit of a liquid sample containing blood according to the electrical resistance it has in response to an alternating electrical potential difference applied across the sample. The measured haematocrit maybe used to improve the accuracy of blood glucose measurements of the blood in another aspect of the invention. It has been found that applying an alternating potential difference (voltage) across such a sample results in a resistance which is surprisingly responsive to haematocrit. The invention exploits this finding. The presence of red blood cells within a blood sample complicates the interpretation of blood glucose measurements using existing methods. The invention may remove or reduce that complication to enable more accurate blood glucose measurements to be made. The alternating potential difference is preferably applied across a gap between two electrodes designed to be bridged by a sample being measured. It has been found that a careful dimensioning of the gap enhances the accuracy of the haematocrit measurement greatly. The gap size is preferably substantially smaller than a gap size between electrodes typically employed in existing systems designed to measure blood glucose. The invention may provide a sampling plate comprising a sample zone for receiving a liquid sample. The sampling plate may have two drive electrodes with separate respective electrode terminals spaced by a spacing for receiving the liquid sample within the sample zone for use in driving an electrical signal through the sample. Two sensing electrodes may be provided with separate respective electrode terminals spaced between the electrode terminals of the two drive electrodes for use in sensing an electrical signal generated by the drive electrodes within a the sample.

Herein, a "sampling plate" may mean any surface capable of receiving a liquid sample in a sample zone. Preferably, however, the sampling plate is portable. Suitably the sampling plate may cover an area less than 1 m², preferably less than 50 cm², more preferably less than 10 cm² and most preferably less than 5 cm² . The sampling plate may cover an area less than 500 mm² - for instance 350 mm² where the sampling plate is 10 mm wide by 35 mm long. Suitably the sampling plate may be rectangular. The sampling plate may be a strip, and may be a flexible strip. Preferably, however, the sampling plate is an individual plate, preferably a rigid sampling plate. The thickness of the sampling plate is preferably less than 1 cm, preferably less than 1 mm, more preferably less than 0.5 mm, most preferably less than 0.25 mm. The sampling plate is preferably compatible with a measurement device. For example, the measurement device is preferably operable to communicate with the sampling plate to measure one or more selected properties of the sample. Preferably the sampling plate may be inserted into the measurement device to allow measurements to be taken.

The two drive electrode terminals may present to each other opposing sides which define between them an elongate gap extending along the sample zone for receiving at least parts of the sample therein. The electrode terminals may be substantially flat and side-by-side to define a substantially flat gap. In a first of its aspects, the invention may provide a sampling plate for use in performing electrical measurements on a liquid sample containing blood comprising a first sample zone for receiving a first liquid sample, a first pair of drive electrodes with separate respective electrode terminals spaced by a spacing for receiving a said first liquid sample within the first sample zone for use in driving an electrical current through the first sample, at least one further sample zone for receiving a further said liquid sample and containing a reagent to react with free glucose in the liquid sample and a further pair of drive electrodes with separate respective electrode terminals spaced by a second spacing for receiving a said further liquid sample within each said further sample zone for use in driving an electrical current through the further sample, wherein the first spacing is less than the second spacing. The first sample zone is preferably adapted for use in measuring the haematocrit of blood in the sample, whereas the at least one further sample zone is preferably adapted for measuring blood glucose levels in the sample.

Preferably, the first spacing defines a gap wider than the average width of a human red blood cell, but less than the average width of two such cells. It is postulated, but not asserted, that the gap serves to form a generally linear array of red blood cells along it in which the array is generally one cell in width - this being constrained by the width of the gap - and that the parts of the gap not occupied by red blood cells are occupied by blood plasma. Blood plasma is typically more electrically conductive than are red blood cells at certain electrical signal frequencies. By applying an oscillating voltage, red blood cells remain mobile (e.g. oscillate) within the gap and may not coat one or other of the electrodes. The result may be that there is maintained within the gap a defined linear array of red blood cells mobile within conductive blood plasma. The proportion of red blood cells within the gap, relative to the quantity of blood plasma there, influences the quantity of electrically conductive pathways (through plasma, around blood cells) available to currents generated by the applied voltage. This may manifest itself as an electrical resistance value determined by a haematocrit value, as has been observed.

The first spacing may be greater than about 80 microns and less than about 300 microns. More preferably, the first spacing is greater than about 120 microns and less than about 200 microns. Yet more preferably, the first spacing is greater than about 150 microns and less than about 170 microns (e.g. about 160 microns). An aforesaid further spacing may be greater than about 300 microns.

The first spacing may be substantially uniform along at least a part of its length. Preferably, the opposing sides/edges of the two drive electrodes are of equal length and oppose each other along substantially their whole lengths. The spacing is preferably linear. This linearity assists in supporting a substantially uniform electric field between the drive electrodes when driven thereby enabling all parts of the drive gap to function in the same way. Manufacture, which may be by printing or etching or laser ablating of electrode patterns onto a substrate, is simplified also. The two drive electrode terminals of one, some or each said pair of drive electrodes may present to each other, across a respective spacing, opposing electrode sides extending along the sample zone which define between them a drive gap for receiving a said sample therein. The drive electrodes may be mounted upon a non-conducting substrate within the respective sample zone and may be adapted for driving an electrical signal transversely across the drive gap through a sample when supported upon the substrate.

Drive electrode terminals of said further pair(s) of drive electrodes may present to each other, across a respective spacing, opposing electrode sides extending along the sample zone. These opposing sides may define between them a drive gap for receiving a sample therein. A further spacing may define a gap which is preferably greater than about 200 microns in width, and may be between 200 microns and 400 microns in width. This dimensioning has been found to be preferable for the electrodes in sampling zones containing the reagent to react with free glucose in the liquid sample. The opposing sides in one, some or each further pair of drive electrodes may be of unequal length. They may be curved. One side may be convex and the opposing side reciprocally concave and of greater length than the convex side. Preferably the electrode with the longer side is used as the cathode of the pair. This is preferable in view of the greater gap size in each further pair of electrodes. It has been found that electrical currents driven across those wider gaps through a blood sample are more prone to diffuse in a direction along the gap rather than flowing directly across the gap un- deviated. In order to better capture diffused charges (current) in the blood sample the electrode to which the charges flow when a direct (DC) voltage is applied between the electrodes, has the longer edge. One, some or each second spacing may be substantially uniform along at least a part of its length.

One, some or each of the drive electrodes may be in electrical communication with a respective electrical contact zone provided on the sampling plate which is exposed for electrical connection simultaneously with an external drive voltage source or with an external sensing circuitry, respectively.

The sampling plate may include two or three separate (or a plurality of) such further sample zones each for receiving a respective liquid sample and each containing a respective reagent to react with free glucose in the liquid sample. Preferably, in each such further sample zone resides a respective pair of drive electrodes with separate respective electrode terminals spaced by a spacing for receiving a respective liquid sample within the further sample zone for use in driving an electrical current through the sample. These may be used for multiple testing, measurement or calibration of blood glucose measurements for enhancing measurement accuracy.

The at least two of, or each of, the sample zones other than the first sample zone are preferably substantially the same in shape and size. Preferably, the at least two of, or each of, the pairs of electrodes within respective sample zones other than the first sample zone are substantially the same in shape and size. The further sampling zones are preferably substantially identical in terms of the features of structure and contents accessible by a blood sample therein. This enables direct comparison (and combination - e.g. for averaging) of blood glucose measurements taken using each of the further sample zones, separately.

In some embodiments, a drive electrode of some or each of the pairs of drive electrodes is a single electrode common thereto. For example, the common electrode maybe the electrode intended for use as a cathode in each electrode pair. Alternatively, it may be the anode. It may be the electrode which, in some embodiments is intended to be grounded (earthed) in use, with a controlled voltage applied to the other electrode of the pair, respectively. This commonality assists in making the manufacture of the sampling plate simpler and cheaper (fewer separate electrode tracks). The material of the electrodes may preferably be Gold (film or sheet), but other conductive elements may be used. The sheet resistance of the electrodes (Ohms per Square) may be between about 2 ohms per square and about 15 ohms per square, preferably about 5 ohms per square.

The reagent carried by the at least one further sample zones may be a deposit formed on one (e.g. exclusively) of the drive electrodes in the sample zone (e.g. the one for use as an anode) to be directly accessible to a sample therein. The deposit may be in the form of an ink or paste. Preferred reagents are oxidising agents. Most preferred are enzymes and especially preferred is glucose oxidase (GOx) or glucose dehydrogenase (GDH).

In a second of its aspects, the invention may provide a sample measurement system comprising a sampling plate as described above, and a measurement device operable to communicate with the sampling plate to measure one or more selected properties of the at least two samples in the first and at least one further sample zones of the sampling plate.

In a third of its aspects, the invention may provide sampling apparatus (e.g. measurement device) for use in performing electrical measurements on a liquid sample containing blood, the apparatus comprising: a first output terminal arranged for outputting an alternating (AC) electrical voltage; and a second output terminal arranged outputting a direct electrical voltage applied thereto (most preferably a substantially constant (DC) voltage) ; and at least one input terminal for receiving an input electrical signal externally input thereto; and a control unit arranged to apply an alternating electrical voltage to the first output terminal and concurrently to measure a first electrical current at the at least one input terminal resulting therefrom when a said liquid sample is in electrical series connection between the first output terminal and the input terminal, and arranged to apply a direct voltage (most preferably a substantially constant electrical (DC) voltage) to the second output terminal and concurrently to measure a second an electrical current at the at least one input terminal resulting therefrom when a liquid sample is in electrical series connection between the second output terminal and an input terminal; and a calculating unit arranged to calculate an electrical resistance value using a value of the first electrical voltage and a value of the concurrently measured first electrical current, and arranged to calculate a first calculated value representing the relative volume of red blood cells in the liquid sample according to the calculated electrical resistance value; and to calculate a second calculated value representing an amount of glucose in the liquid sample according to both the first calculated value and the measured second electrical current, and to output the result.

The control unit may be arranged to apply the alternating electrical voltage and the direct voltage concurrently (i.e. both at the same time). This has the benefit of allowing both the AC and the DC signals and measurements to be made quickly which is important because blood samples will degrade rapidly once they have been removed from the body and placed in a sampling plate. It is highly preferable to perform all sampling measurements on the sample as quickly as possible to ensure accuracy of the measurements by limiting errors which could be introduced by rapid degradation of the sample over time. Nevertheless, the control unit may arranged or operable to apply the alternating electrical voltage and the direct electrical voltage separately and in succession if desired. This can be acceptable if the successive measurements are each made sufficiently rapidly and in sufficiently rapid succession. The apparatus may include a first voltage unit in electrical communication with the first output terminal for applying thereto an alternating (AC) electrical voltage. The apparatus may include a second voltage unit in electrical communication with the second output terminal for applying thereto a direct voltage (most preferably a substantially constant (DC) electrical voltage). The control unit may control the first and second voltage units to apply their respective voltages.

The apparatus may include one or more electrical current detector(s) in electrical communication with the at least one input terminal (e.g. respectively), being arranged for measuring an electrical current according to a received said input electrical signal. The control unit may control operation of the current detector(s).

The calculating unit may be arranged to produce, according to the measured second current, a stored value representing an amount of glucose in the sample (e.g. free glucose which is un-associated with red blood cells). The second calculated value may comprise the product of the first calculated value and the stored value.

The sampling apparatus may include a memory unit arranged to retrievably store a plurality of these stored values each in association with a respective measured second current value. The calculating unit may be arranged to produce a given such stored value by retrieving it from the memory unit according to a measured second current value with which it is associated. Thus, pre-stored calibration values or curves may be stored in the memory unit whereby a newly-measured second current value can be directly associated with a pre-stored blood glucose value and retrieved from the memory unit to provide a blood glucose value associated with that particular second current measurement. The calibration curves may be analytical curves (e.g. in equation form) representing the calibration curve(s) and the relationship between measured current and blood glucose, or may be tabulated value in a look-up table.

A sampling apparatus may include a plurality of second output terminals and the control unit may be arranged to apply thereto a respective direct (e.g. substantially constant) DC electrical voltage. The control unit may include a second voltage unit in electrical communication with each of the second output terminals for applying the voltage to them. The control unit may be arranged to control the second voltage unit to apply a respective direct (e.g. substantially constant) electrical voltage and concurrently to control the electrical current detector(s) to measure a respective second electrical current resulting therefrom when a respective liquid sample is in electrical series connection between the respective second output terminal and an input terminal.

The calculating unit may be arranged to calculate the second calculated value representing an average of the amount of glucose in a plurality of the liquid samples according to both the first calculated value and the plurality of measured second electrical currents, and to output the result.

The sampling apparatus may include a socket adapted for receiving electrode terminals of a sampling plate wherein the first output terminal, the second output terminal and the at least one input terminal(s) are exposed within the socket for forming simultaneously an electrical contact with a plurality of the electrode terminals of a sampling plate when received in the socket.

The sampling apparatus described above may include the sampling plate described above. For example, one drive electrode of said first pair of drive electrodes of the sampling plate may be adapted to disconnectably connect electrically to the first output terminal of the sampling apparatus and the other drive electrode of the first pair of drive electrodes is adapted to disconnectably connect concurrently to an input terminal of the sampling apparatus. One drive electrode of the second pair of drive electrodes of the sampling plate may be adapted to disconnectably connect electrically to the second output terminal of the sampling apparatus and the other drive electrode of the second pair of drive electrodes of the sampling plate may be adapted to disonnectably connect concurrently to an input terminal of the sampling apparatus. This arrangement may thereby connect the first pair of drive electrodes and the second pair of drive electrodes of the sampling plate to the sampling apparatus simultaneously for electrical communication therewith.

In a fourth aspect, the invention may provide a sampling apparatus for use in performing electrical measurements on a liquid sample containing blood, the apparatus comprising: an output terminal for outputting an alternating electrical voltage (AC) applied thereto; and an input terminal for receiving an input electrical signal externally input thereto; and a control unit arranged to apply an alternating electrical voltage signal to the output terminal and concurrently to measure an electrical current resulting therefrom when a liquid sample is in electrical series connection between the output terminal and the input terminal; a calculating unit arranged to calculate an electrical resistance value using a value of the electrical voltage and of the concurrently measured electrical current, and to calculate a value representing the relative volume of red blood cells in the liquid sample according to the calculated electrical resistance value, and to output the result.

The sampling apparatus may include a voltage unit in electrical communication with the output terminal for applying thereto an alternating electrical voltage. The control unit may control the voltage unit to apply the voltage. The sampling apparatus may include an electrical current detector in electrical communication with the input terminal, which may be controlled by the control unit to measure the electrical current.

The calculating unit of a sampling apparatus as described in any aspect above, may be arranged to calculate said first calculated value (HCT) according to the electrical resistance value (R) using the following equation:

where A is a constant with a value in the range from about 20 ohms to about 300 ohms, and B is a constant with a value in the range from about 40 ohms to about 250 ohms.

The frequency of the alternating electrical signal preferably has a value in the range 1 KHz to 150KHz. More preferably, the frequency of said alternating electrical signal has a value in the range 25KHz to 125KHz, yet more preferably the frequency of said alternating electrical signal has a value in the range 35KHz to 100KHz, even more preferably the frequency of said alternating electrical signal has a value in the range 45KHz to 75KHz, yet even more the frequency of said alternating electrical signal has a value in the range 47KHz to 53KHz. It has been found that a frequency of about 50KHz works especially well, and frequencies reasonably close to this value are desirable, though the ranges given above have been found to be acceptable in terms of accuracy of measurement in implementing the invention. In general, the preferred range of frequencies, and the preferential frequency within such a range, is influenced to some extent by geometrical considerations of the sampling process. Factors such as the size of surface area of conductive elements/electrodes within a test area of a sampling plate, in relation to the size of surface area of non-conductive/non-electrode parts between electrodes, can influence the position and extent of the suitable AC signal frequency ranges. These surface areas may typically be located within a sampling area, well or zone within a sampling plate which is between about 0.5mm and 5mm in diameter or width, or more preferably between about 1 mm and 3mm, such as about 1 .6mm in diameter or width. These dimensions enable a sample size which is large enough to do reliable measurement upon, but does not result in a sampling size (or sampling plate size) which is too large for these purposes, or for practical use generally. The alternating electrical voltage preferably has an amplitude in the range from about 0.05 volts to about 2 volts. The direct voltage may be a substantially constant (DC) voltage which is preferably a substantially constant potential difference between the second output terminal and a said at least one input terminal. The alternating (AC) voltage is preferably an alternating potential difference between the first output terminal and a said at least one input terminal.

In a fifth aspect, the invention may provide a sample measurement method for performing electrical measurements on a liquid sample containing blood, the method comprising: receiving the liquid sample on a sample plate comprising first electrode terminals separated by a first spacing adapted to be bridged by blood from the liquid sample; and, applying to the electrodes an alternating electrical voltage having a given signal frequency to generate a first alternating potential difference across the first spacing; and, measuring a first electrical current passing across the first spacing and therefrom determining a value of an electrical resistance of the liquid sample bridging said first spacing, and determining a relative volume of red blood cells in the liquid sample using said resistance value; and, receiving the liquid sample on the sample plate comprising second electrode terminals separated by a second spacing adapted to be bridged by blood from the liquid sample; and, applying to the electrodes a direct voltage (most preferably a substantially constant (DC) voltage) to generate a second direct (most preferably substantially constant) potential difference across the second spacing; and, measuring a second electrical current passing across the second spacing; and, determining an amount of glucose in the liquid sample bridging said second spacing using the measured value of the second electrical current and the value of the relative volume of red blood cells in the liquid sample.

In a sixth aspect, the invention may provide a sample measurement method for performing electrical measurements on a liquid sample containing blood, the method comprising: receiving the liquid sample on a sample plate comprising electrode terminals separated by a spacing adapted to be bridged by blood from the liquid sample; and, applying to the electrodes an alternating electrical voltage having a given signal frequency to generate an alternating potential difference across the spacing; and, measuring an electrical current passing across the spacing and therefrom determining a value of an electrical resistance of the liquid sample bridging said spacing; and, determining a relative volume of red blood cells in the liquid sample using said resistance value. To better illustrate the invention there now follows a non-limiting examples of embodiments of the invention with reference to the accompanying drawings of which:

Figure 1 illustrates schematically a sampling plate according to a first embodiment of the invention, in the form of a disposable sampling strip; Figure 2 illustrates schematically the sampling plate of figure 1 attached to a sampling unit according to an embodiment of the invention;

Figure 3 illustrates schematically a sampling plate according to a second embodiment of the invention, in the form of a disposable sampling strip comprising four sampling zones;

Figure 4 illustrates schematically the sampling plate of figure 3 attached to a sampling unit according to an embodiment of the invention;

Figure 5 illustrates schematically parts of a the sampling plate of the form of the plate of figure 3;

Figure 6 schematically illustrates schematically the form of a time-dependent current passed through a sample in a sampling zone of the sampling plate of figure 3;

Figure 7 schematically shows a first example of an ASIC and other circuitry components adapted to implement signal generation and reception to and from a sampling plate of the form of figure 3;

Figure 8 schematically shows a second example of an ASIC and other circuitry components adapted to implement signal generation and reception to and from a sampling plate of the form schematically illustrated in figure 8;

Figure 9 schematically shows a first example of an ASIC and other circuitry components adapted to implement signal generation and reception to and from a sampling plate of the form of figure 3;

Figure 10 illustrates schematically a sampling plate according to a variant of the first embodiment of the invention, in the form of a disposable sampling strip.

In the drawings, like items are assigned like reference symbols.

Figures 1 and 10 each show a sampling plate (1 , 1 10) in the form of a strip of firm and non-conductive material (e.g. plastic) possessing a circular sample zone (2) defined by a circular recess formed within the strip for receiving a liquid blood sample. Within the sample zone there are two electrode terminals (3, 4) formed upon a surface of the plate forming the floor of the sample zone and exposed for contact with a received sample. The electrode terminals each comprise a layer of inert conductive material, preferably Gold. The layer may be printed, or laid down by a sputter process or other method as would be readily apparent to the skilled person.

The two electrode terminals comprise a pair of drive electrode terminals (3, 4) each of which is in the shape of a circular segment the curved edge of which coincides with a part of the circular edge of the circular sample zone. The straight segment edge of each one of the two drive electrode terminals is parallel to and opposes the straight segment edge of the other of the two drive electrode terminals to define between them a straight, elongate drive gap (5) of uniform width within the sample zone across which the drive electrode terminals oppose each other and across which a drive voltage is applied as explained in more detail below. The width of the drive gap is preferably between about 80 microns and about 300 microns, but in preferred embodiments is about 160 microns and is dimensioned to admit, at any point along the drive gap, a single human blood cell without permitting that blood cell to bridge the drive gap and concurrently contact both of the two drive electrodes defining the drive gap. Rather, the drive gap is dimensioned to allow one blood cell space to oscillate within the gap between the opposing sensing electrodes in response to an alternating drive voltage driven transversely across the drive gap between the two drive electrode terminals (3, 4). In this way, a row of blood cells may be arranged along the drive gap when a liquid blood sample is received within the sensing zone and may be subject to an alternating drive voltage directed transversely (e.g. substantially perpendicular) to the row of cells. This geometry, and the linear array of single blood cells it enables in use, has been found to provide a surprisingly accurate and stable means of measuring haematocrit (HCT) values for the blood sample. Accurate determination of HCT has been found to be important for enabling accurate determination of levels of glucose within the plasma of a blood sample - i.e. one preferably should know how much of the blood sample is comprised of red blood cells in order to be able to estimate the quantity of glucose in the rest of the blood (i.e. the plasma), although this estimate may be influenced by any viral infections (e.g. influenza) carried within the sample. The present invention is able to determine HCT in order to permit a measurement of blood plasma glucose levels with reliability and accuracy.

Figure 1 illustrates an embodiment of a sensing plate in the form of a disposable strip (1). The electrode and conductor structure of the disposable strip is as described above with reference to Figure 1 . In addition, at the end of the strip containing the drive electrode terminals (3, 4), there is an air-porous body (27) which is in fluid communication with the sample zone wherein the air porous body is arranged to receive air displaced from the sample zone as the liquid blood sample is received into the sample zone.

The term "In fluid communication with" may mean interfacing, where "interfacing" means sharing a common boundary. Preferably "in fluid communication with" refers to where the air porous body is adjacent to the sample zone. The air porous body may define a floor of the sample zone and/or wall(s) of the sample zone. The air porous body may surround the sample zone. Preferably the air porous body defines the sample zone, or defines an outer boundary of the sample zone. Preferably the air porous body defines the perimeter of the sample zone or at least part of the perimeter of the sample zone. Preferably the air porous body is external to the sample zone itself. Preferably the sample zone is free of air porous body.

Preferably the air porous body is arranged to receive displaced air as the liquid sample approaches the air porous body. Preferably the air porous body is arranged to receive air displaced in the same direction as the liquid sample travels (or spreads) into the sample zone. Preferably the air porous body is arranged to receive a side-ways displacement of air as the liquid sample approaches the air porous body in a side-ways manner. Preferably the sample zone is arranged to prevent back flow of the liquid sample. An advantage of this arrangement is that the air porous body helps the liquid sample to flow into the sample zone with minimal air resistance, by providing a means by which air can be directly displaced - preferably in the same direction as the liquid sample enters the sample zone. This permits the liquid sample to enter the sample zone at a faster rate. In contrast, where such an air porous body is absent, air resistance retards the flow of the liquid sample into the sample zone. Another advantage of the arrangement is that the air porous body helps the liquid sample to spread uniformly throughout the sample zone, thus giving greater sampling consistency and consequently more accurate measurements. In contrast, where the air porous body is absent, air resistance affects the fluid dynamics of the liquid sample by discouraging spreading (air resistance from all sides) and instead encouraging the liquid sample to remain collectively associated as a bulk (aided by surface tension). As such the liquid sample tends to flow as a bulk in a single direction since in this way the bulk overcomes air resistance in that particular direction. Another advantage is that formation of air- pockets is alleviated, which again allows for better spreading and more accurate measurements. The liquid sample is preferably hydrophilic, more preferably aqueous-based, and most preferably blood. In this case, blood glucose levels of a diabetic patient may be measured. The air porous body is preferably substantially impermeable to the liquid sample. The air porous body is preferably substantially impermeable to water. The air porous body is preferably substantially impermeable to an aqueous liquid sample, and most preferably substantially impermeable to blood.

The air porous body is preferably located substantially around the perimeter of the sample zone. Preferably a floor of the sample zone is free of air porous body. Preferably the sample zone is free of a roof. Where the sample zone comprises a roof, the roof is preferably free of air porous body. The air porous body preferably comprises hydrophobic material. Preferably the air porous body comprises at least 50 wt%, more preferably at least 70 wt%, and most preferably at least 90 wt% hydrophobic material. The air porous body preferably has an average pore size between 10 and 300 microns, preferably between 50 and 200 microns, and most preferably between 100 and 150 microns. The air porous body preferably comprises an air porous mesh, which again is preferably hydrophobic overall. Such an air porous mesh preferably comprises polyether ether ketone (PEEK), polypropylene (PP), polyester (PET), polyvinylidene fluoride (PVDF), ethylene chlorotrifluoroethylene (ECTFE), ethylene co-tetrafluoroethylene (ETFE), nylon (polyamide), or fluorinated ethylene-propylene (FEP). The air porous mesh preferably comprises polyester (PET). Most preferably the air porous mesh comprises Sefar 07-120 34. Accordingly, where the sample zone (2) has a roof, the sample zone is accessible via an entry port (25) into which a blood sample (26) maybe placed. By capillary action, the blood sample is drawn through the entry port and into the sampling zone, displacing air into the air-porous body (27) as it does so, to finally occupy the sample zone covering the drive and sensing electrode terminals there. A breathable structure created beneath a thin polymer film covering the sample zone, as a roof. Typically the porous layer is a mesh made up of strands of polymer that are coated to create a hydrophobic boundary to the blood as it flows on to the sample zone. A geometric shape cut into the mesh defines the sample zone and entry port which allows the sample to fill the sample zone under capillary action created by the thin top film. Each of the electrode terminals (3, 4) within the sensing zone is electrically connected to a respective electrical conductor line (6, 7) formed within the body of the sensing plate so as to be electrically insulated along its length until terminating at a respective exposed electrical contact zone (8, 9) at an end or side of the sampling plate distal from the sample zone. For example, a first drive electrode terminal (3) is electrically connected to a first drive contact zone (9) via a first (7) electrical conductor strip (e.g. Gold). The second drive electrode terminal (4) is electrically connected to a second drive contact zone (8) via a second (6) electrical conductor strip (e.g. Gold). These two contact zones are arrayed in a line along an edge of the sensing plate, at the distal end of the strip, to permit the end of the strip to be inserted into an electrical socket/port of an electrical sensing unit (15, Fig. 2) to place each one of the two contact zones simultaneously in electrical connection with a respective one of two (16, 17) sensing contact terminals of the sensing unit. Figure 2 schematically illustrates this in which the sensing unit (15) may be a handset, or part of a larger piece of equipment. The sensing unit comprises a drive voltage unit (18) arranged to generate an alternating electrical voltage of selected amplitude and selected frequency, and apply the alternating voltage between the first and second sensing contact terminals (16, 17) for application to the drive electrode terminals (3, 4) as a drive voltage via the first and second drive contact zones (8, 9) of the sensing plate. An electrical current detector unit (20) is electrically in communication with one of the first and second contact terminals (16, 17) for receiving, via the first and second sensing contact zones (8, 9) of the sensing plate, an electrical current that has passed between the first and second drive electrode terminals (3, 4) as a result of the drive voltage applied between them by the drive voltage unit. The current detector unit is arranged to detect and measure the amplitude of the received alternating (AC) current. A control unit (19) is operatively connected to the drive voltage unit (18) to control the frequency, amplitude and production of the generated drive voltage signal. For example, the control unit may control the voltage signal frequency to be a value within a first continuous range of values from about 1 KHz to about 150 KHz, or to be a value within a second continuous range of values from about 10KHz to about 100 KHz. More preferably, the frequency has a value in the range 25KHz to 125KHz, yet more preferably in the range 35KHz to 100KHz, even more preferably in the range 45KHz to 75KHz, yet even more preferably the frequency has a value in the range 47KHz to 53KHz. Most preferably, the frequency is controlled to be about 50 KHz. The voltage amplitude may be controlled to be in the range from about 0.05 Volts to about 2 Volts, preferably about 0.25 Volts to 1 .0 Volts, such as 0.4 Volts (400 mV).

The control unit is connected in communication with the electrical current detector unit (20) to communicate thereto control signals. The control unit (19) is arranged to control the voltage unit to apply an alternating electrical drive voltage at a given frequency (selected from within the above range of values) and concurrently to control the current detector unit (20) to measure an amplitude of an electrical current signal received via the drive electrodes. A calculating unit (21) is connected to the control unit for communication therewith and is arranged to receive data signals and control signals from the control unit. The calculating unit is arranged to receive from the control unit a value of the amplitude of the drive voltage applied across the drive terminals (3, 4) by the voltage unit, and to receive from the current detector unit (e.g. via the control unit) a concurrently measured value of the amplitude of the AC current detected thereby which result from the driving of the AC drive voltage across a blood sample when bridging the drive gap (5). Upon receiving a command from the control unit, the calculating unit is arranged to calculate a value of the electrical resistance (R) of the blood sample bridging the drive gap across which the AC drive voltage (amplitude V volts, at the given drive frequency) has been driven and through which an AC current (amplitude I, amps) consequentially flowed. The calculation is performed according to the formula:

R = , ohms.

The calculating unit is further arranged to then calculate a % value (HCT) for the haematocrit of the blood sample according to the formula:

A

HCT = 100 x 1

(R + B) where A is a constant with a value in the range from about 20 ohms to about 300 ohms, and B is a constant with a value in the range from about 40 ohms to about 250 ohms. With Gold sheet having a sheet resistance of 5 Ohms per Square used for the drive electrodes (3, 4), and a drive gap (5) size of 160 microns, the values of A and B have been found to be: A = 149.1 Ohms; B = 107.4 Ohms. The values of A and B vary in dependence upon the value of the sheet resistance (Ohms per Square) of the drive electrode material, the surface area of the drive electrodes accessible to the blood sample within the sampling zone - for example, the ratio of how much of the sample sits upon the electrodes and how much sits in the drive gap - and, the wetting ability of the electrode material with respect to the blood sample. This is influenced by the electrode material (e.g. Gold), and the quality of the structure (e.g. roughness) of the electrode surfaces. These factors may vary from one implementation of the sampling strip to another, however, the ranges of given above for the values of A and B are representative. A calibration operation, using two commercially available calibration blood samples of different known HCT values, would be sufficient to enable values of A and B to be determined from respective resistance (R) measurements using a given sampling plate, as the skilled person within this art will readily appreciate. Using the method implemented by the apparatus described above, the standard error of the estimated HCT values, when compared against the known micro-heamatocrit method, is found to be less than 1 .5% when measuring HCT in the range of 20 to 60%. Results shown in Table 1 below illustrate five HCT measurements (new strip each time) according to the invention for each of four different HCT values using blood samples with known (different) HCT levels. The average blood plasma glucose level was 62mg/dl in the samples, and the blood type was expositive.

Table 1

Strip No. Resistance @ 50KHz Actual HCT Value Measured HCT Value

1 326 65% 65.5976

2 325.3 65.54195

3 326.3 65.6214

4 325.7 65.57377

5 325.4 65.54991

Strip No. Resistance @ 50KHz Actual HCT Value Measured HCT Value

1 242 56% 57.32685

2 241.6 57.27794

3 241.9 57.31463

4 242.2 57.35126

5 241.7 57.29017

Strip No. Resistance @ 50KHz Actual HCT Value Measured HCT Value

1 195.3 50% 50.74331

2 193.1 50.3827

3 194.3 50.58005

4 194.3 50.58005

5 193.6 50.46512

Strip No. Resistance @ 50KHz Actual HCT Value Measured HCT Value

1 105.4 29% 29.93421

2 106.2 30.19663

3 105.3 29.90127

4 105.7 30.03285

5 105.1 29.83529

Before measurement of a sample of blood is performed, the temperature of the sampling plate may be established. This may be done by any suitable means such as would be available and apparent to the skilled person. Preferably, the temperature of the sampling plate may be determined means of a thermocouple mounted on the sampling strip and arranged for electrical connection to the control unit if the sensing unit which may be adapted to measure the temperature of the sampling strip and display the result to the user. To further improve accuracy the temperature of the sample is preferably be maintained at a constant temperature during sampling, and that temperature may be a predetermined optimal temperature (e.g. 37°C ± 1 .5°C). This may be achieved, for example, by environmental temperature control of the area in which sampling plates are stored or used, or by means of a heater (e.g. trace heating, ohmic heating wire/strip etc, not shown) formed in the strip electrically connectable to a power source within the sensing unit (15, Fig.2) to controllably heat the sampling plate. A thermocouple (not shown) may also be formed within/upon the sampling plate, also being arranged to be powered by the sampling unit when the sampling plate is connected thereto in use. This may be used to regulate the heater (if present) and/or simply to allow the sampling unit to determine the temperature of the sampling plate.

It has been found that the presence of red blood cells in the blood sample has a measurable effect upon the magnitude of alternating (AC) electrical current signal passing through a blood sample when subjected to a given AC potential difference (voltage). The presence of these red blood cells therefore can be considered as influencing the resistance R of the blood sample attributable to the plasma within the sample. Relatively large populations of red blood cells reduce the relative proportion of plasma in the blood sample and thereby reduce the effectiveness of that conductive pathway for electrical currents driven through the sample, thereby increasing electrical resistance, and vice versa. It is postulated, though not asserted, that the membrane of a red blood cell forms an insulator which inhibits electrical current from passing through the cell. Thus, the higher the density of red blood cells, the higher will be the resistive components of the blood sample caused by the red blood cell membranes, given that the blood plasma will provide the main conductive pathway through the sample.

A drive gap of about 160 microns of a substantially uniform width is suitable for this purpose. Each of the two drive electrode terminals (3, 4) is a flat segment of Gold film or sheet. The sheet resistance (Ohms per Square) of the drive electrodes is about 5 Ohms per Square. Gold sheet produced by Mitsubishi is suitable. Sheet resistances of as low as 2 Ohms per Square or as high as 15 Ohms per Square may be used.

In an alternative embodiment, the control unit may be arranged to apply the alternating electrical voltage and a direct (DC) voltage concurrently (i.e. both at the same time). In such a case, the control unit may be adapted or arranged to operate/communicate with the sampling plate design illustrated in Figure 10. This sampling strip (1 10) corresponds to the structure and function of the sampling plate (1) illustrated in Figure 1 and described above, with the additional feature of a DC probe electrode

(1 1 1) having a terminal located substantially centrally within the drive gap (5) equidistant between the two drive electrode terminals (3, 4) within the sample zone. The DC probe electrode is electrically connected to a respective electrical conductor line formed within the body of the sampling strip so as to be electrically insulated along its length until terminating at an exposed electrical contact zone

(1 12) at an end or side of the sampling plate distal from the sample zone. For example, a DC probe electrode may be Gold. Thus, three contact zones (8, 9, 1 12) are arrayed in a line along an edge of the sensing plate, at the distal end of the strip, to permit the end of the strip to be inserted into an electrical socket/port of an electrical sensing unit (15, Fig. 2) to place each one of the three contact zones simultaneously in electrical connection with a respective sensing contact terminal of the sensing unit. In this regard, the sensing unit illustrated in Figure 2 would include a DC drive voltage unit (not shown) arranged to generate direct (DC) electrical voltage of selected amplitude and apply the DC voltage to the DC probe electrode contact terminal (1 12) for application to the DC probe electrode terminals (1 1 1) as a DC drive voltage via the third DC drive contact zone (1 12) of the sensing plate. In this way a DC voltage may be applied to the sample zone, between the AC drive electrodes (3, 4) as and when desired, in order to apply a DC bias voltage to the sample there. Thus may be done concurrently with application of the AC signal, or separately (e.g. before/after) from applying the AC signal to the sample. The use and benefit of applying a DC voltage to the sample may be as described below, in a further embodiment, in which separate sample regions are provided in a sampling plate (e.g. Figure 3). It will be appreciated that a single sample region may be provided in alternative plate design, such as shown in Figure 10, for implementing the methodology described below. Figure 3 illustrates another embodiment of the invention for use in measuring the amount of glucose in the plasma component of a blood sample. In this embodiment, the sampling strip (30) comprises four pairs of separate drive electrodes. These four pairs consist of a first pair (31 , 32) of drive electrodes separated by a flat, linear drive gap (33) of uniform width of the same structure as the electrodes and drive gap (3, 4, 5) of Figure 2. A second pair (37, 38) of drive electrodes is separated by a flat, curved drive gap (34) of uniform width in which one concave edge of the curved drive gap is longer than the other reciprocally convex edge of the drive gap on the opposing electrode of the pair. A third pair (39, 40) of drive electrodes is similarly separated by a flat, curved drive gap (35) of uniform width in which one concave edge of the curved drive gap is longer than the other reciprocally convex edge of the drive gap on the opposing electrode of the pair. And again, a fourth pair (41 , 42) of drive electrodes is separated by a flat, curved drive gap (36) of uniform width in which one concave edge of the curved drive gap is longer than the other reciprocally convex edge of the drive gap on the opposing electrode of the pair. The drive gaps of the second to fourth electrode pairs have the same shape and dimensions. The width of the drive gap of the first pair of electrodes is about 160 microns in size while the width of the drive gap of the second to fourth electrode pairs is about 300 microns in size.

The electrode of each of the second to fourth electrode pairs bearing the concave (longer) edge, and one of the two electrodes of the first pair, are electrically connected to (integrally formed with) a single common conductive strip (43) which terminates at a contact point (51 ) at the distal end of the sampling strip. These electrodes may serve as cathodes. The other electrodes of each of the four pairs of electrodes are electrically connected to (integrally formed with) separate respective conductive strips (44, 45, 46, 47) which each terminate at a respective contact point (48, 49, 50, 52) at the distal end of the sampling strip. These electrodes may serve as anodes. A deposit of Glucose oxidase (GOx) is located upon the convex-edged electrode terminal (38, 40, 42) of each of the second to fourth electrode pairs. The deposits are shown as circular dots on the electrodes of Figure 3.

The four electrode pairs are exposed within a respective four sampling zones (Figure 5) for receiving a blood sample. The sampling zones are not shown in Figure 3 for clarity. Figure 5 shows the sampling strip (30) of Figure 3 in more detail, in which the electrode and conductive strip structures of Figure 3 are omitted for clarity, but in which overlying structures are shown defining the sampling zones. There is an air-porous body (71) which is in fluid communication with four separate sample zones (73 to 76) wherein the air porous body is arranged to receive air displaced from each sample zone as the liquid blood sample is received into the sample zone. A series of hydrophobic ink deposits (79) are arranged at the openings to each of the four sampling zones, as well as between sampling zones, to repel blood therefrom and to restrain blood within the sampling zones. Each of the four sampling zones is in fluid communication with a sample inlet opening (72) at the end of the sampling strip. The four sampling zones form a rectangular array with each sampling zone in register with a respective one of the four pairs of drive electrodes illustrated in Figure 3. Two sampling zones are arranged to one side of a sample conduit and the two other sampling zones are arranged to the other side of the conduit. The conduit extends from the sample inlet opening (72), past the sampling zones, and terminates at a sample reservoir (77) containing a wicking material designed to draw into the sample reservoir any excess sample which has passed along the conduit from the sample inlet opening and has not entered one of the four sampling zones. In this way, one sample of blood may be applied in common to all four of the sampling zones in one simple operation.

A thermo-couple (53) is formed on the sampling strip between two of the conductive strips connected to electrodes, and extends from a contact point at the distal edge of the strip to a point about mid-way along the length of the sampling strip approximately along the centre of the strip. This thermo-couple is operable to provide temperature information to a sensing to which the distal end of the sampling strip is arranged to connect physically and electrically into a dedicated slot (plug-and-socket). The array of contact points (48 to 53) are evenly spaced from each other and equally adjacent the distal strip edge to permit electrical connection to reciprocal electrical contacts formed in the socket of the sensing unit. Alternatively, the thermo-couple may be formed within the sampling unit and be arranged to make physical contact with the sampling strip when the sampling strip is connected to the sampling device in use. Figure 9 shows an example of this.

Figure 4 schematically illustrates this in which the sensing unit (60) may be a handset, or part of a larger piece of equipment. The sensing unit comprises a first drive voltage unit (61) arranged to generate an alternating electrical voltage of selected amplitude and selected frequency, and apply the alternating voltage between first and second sensing contact terminals (64, 65) for application to the drive electrode terminals (31 , 32) of the first pair of drive terminals as a drive voltage via the first and second drive contact zones (51 , 52) of the sensing plate (30). A first electrical current detector unit (67) is electrically in communication with one of the first and second contact terminals (64, 65) for receiving, via the first and second sensing contact zones of the sensing plate, an electrical current that has passed across the first drive gap (33) as a result of the drive voltage applied there by the drive voltage unit. The first current detector unit is arranged to detect and measure the amplitude of the received alternating (AC) current.

The sensing unit comprises a second drive voltage unit (62) arranged to generate an constant (DC) electrical voltage of selected value and apply it between the second sensing contact terminals (65) and each of the contact terminals (48, 49, 50) associated with the convex-edged drive electrode terminals of the second to fourth pairs of drive electrodes (38, 40, 42) for application to the drive electrode terminals (37 to 42) of the second to fourth pairs of drive terminals as a drive voltage. A second electrical current detector unit (66) is electrically in communication with contact terminals associated with the second to fourth pairs of drive electrodes, for receiving an electrical current that has passed across a respective one of the second to fourth drive gap (34, 35, 36) as a result of the drive voltage applied there by the second drive voltage unit. The second current detector unit is arranged to detect and measure the received DC current.

A control unit (63) is operatively connected to the first and second drive voltage units (61 , 62) to control the frequency, amplitude etc and production of the generated drive voltage signals. For example, the control unit may control the first voltage signal frequency to be a value within a first continuous range of values from about 1 KHz to about 150 KHz, or to be a value within a second continuous range of values from about 10KHz to about 100 KHz. More preferably, the frequency has a value in the range 25KHz to 125KHz, yet more preferably in the range 35KHz to 100KHz, even more preferably in the range 45KHz to 75KHz, yet even more preferably the frequency has a value in the range 47KHz to 53KHz. Most preferably, the frequency is controlled to be about 50 KHz. The voltage amplitude may be controlled to be in the range from about 0.05 Volts to about 2 Volts, preferably about 0.25 Volts to 1 .0 Volts, such as 0.4 Volts (400 mV). The DC voltage may be controlled to be within the range of about 0.1 volts to about 1 .0 volts, or preferably between 0.2 volts and 0.8 volts or more preferably between 0.3 volts and 0.7 volts - for example, about 0.4 Volts (400 mV).

The control unit is connected in communication with the electrical current detector units (66, 67) to communicate thereto control signals. The control unit (63) is arranged to control the voltage units to apply an AC or DC electrical drive voltage at a given frequency (selected from within the above range of values) and concurrently to control the current detector units (66, 67) to measure an amplitude/magnitude of an electrical current signal (AC/DC) received via the drive electrodes. A calculating unit (within control init 63) is controlled by the control unit and is arranged to receive data signals and control signals from the control unit. The calculating unit is arranged to receive from the control unit a value of the amplitude of the AC drive voltage applied across the drive terminals of the first pair of drive electrodes by the first voltage unit, and to receive from the first current detector unit (67) a concurrently measured value of the amplitude of the AC current detected thereby which result from the driving of the AC drive voltage across a blood sample when bridging the drive gap between the first pair of drive electrodes. Upon receiving a command from the control unit, the calculating unit is arranged to calculate a value of the electrical resistance (R) of the blood sample bridging the first drive gap across which the AC drive voltage (amplitude V volts, at the given drive frequency) has been driven and through which an AC current (amplitude I, amps) consequentially flowed. The calculation is performed according to the formula:

R = , ohms.

A

HCT = 100 x 1

(R + B) where A is a constant with a value in the range from about 20 ohms to about 300 ohms, and B is a constant with a value in the range from about 40 ohms to about 250 ohms. With Gold sheet having a sheet resistance of 5 Ohms per Square used for the drive electrodes (31 , 32), and a drive gap (33) size of 160 microns, the values of A and B have been found to be: A = 149.1 Ohms; B = 107.4 Ohms. The values of A and B vary in dependence upon the value of the sheet resistance (Ohms per Square) of the drive electrode material, the surface area of the drive electrodes accessible to the blood sample within the sampling zone - for example, the ratio of how much of the sample sits upon the electrodes and how much sits in the drive gap - and, the wetting ability of the electrode material with respect to the blood sample. This is influenced by the electrode material (e.g. Gold), and the quality of the structure (e.g. roughness) of the electrode surfaces. These factors may vary from one implementation of the sampling strip to another, however, the ranges of given above for the values of A and B are representative. A calibration operation, using two commercially available calibration blood samples of different known HCT values, would be sufficient to enable values of A and B to be determined from respective resistance (R) measurements using a given sampling plate, as the skilled person within this art will readily appreciate. As described above with reference to Figure 3, a deposit of an enzyme, or a glucose oxidaze ("GOx") or glucose dehydrogenase ("GDH"), is located upon the anode of the two drive electrode terminals in each of the second to fourth sample zones. The enzyme (or GOx GDH) is placed to allow it to make contact with, and react with, a blood sample entered into the second, third and fourth sample zones. In doing so, the enzyme (or GOx GDH) reacts with the blood sample to oxidise free glucose present within the plasma of the blood sample. The oxidation of glucose has been found to directly influence the electrical resistance of the blood sample over time as the oxidation process proceeds.

It has been found that a direct (DC) voltage held between the two drive electrodes of any of the second to fourth sample zones, and thereby applied across the blood sample located there during the enzyme/GOx/GDH reaction period, will cause a time-varying current to pass through the blood sample. Variation is believed to result, in part, from the changing (falling) quantity of free glucose within the plasma of the sample resulting in a changing electrical resistance of the plasma component of the blood. Most preferably, the DC voltage is substantially constant for simplicity, however non- constant DC voltages (e.g. smoothly falling or rising in a controlled way) could be employed if desired, though this is likely to complicate design and operation of the apparatus and so a substantially constant DC voltage is preferred.

Figure 6 schematically illustrates the form and shape of the changing electrical current typically seen under such circumstances. Starting with an initial rise (near instantaneous) in current to a peak value at a time "t_peak", the observed quantity of current falls monotonically as glucose is increasingly oxidised in the blood plasma. Sample temperature affects the rate of decay of the current - lower temperatures result in faster decay. It has been found that the rate of fall of the observed current, following the peak current value, is characteristic of the amount of glucose originally present in the plasma of the blood sample before the oxidisation process began. The observed current decay is highly reproducible when the process is repeated. Thus, by performing this process initially with a sufficient plurality a blood samples each having an incrementally different, known quantity of glucose in its plasma component, one may build-up a plurality of reference curves of the type shown in Figure 6 (or data sets representative of them) from which a future blood plasma glucose measurement may be made by reference. That is to say, with the plurality of reference curves (or representative data) one may perform a current blood sampling operation as described above so as to generate a current varying generally according to the current decay curve as shown in Figure 6. By measuring a particular current value at a selected time during that current decay (i.e. a point along the contemporaneous current decay curve) one may subsequently identify a blood plasma glucose level associated with that current value as derived from a reference curve. The contemporaneously measured blood plasma glucose level may then be concluded to have the same glucose level. A Look-Up Table (LUT) or other storage may be used for this purpose. The process may include measuring a contemporaneous value "l_m" of the decaying current at a specified time "t_m" following the time "tpeak" at which the detected peak of the measured current occurs - the specified time having also been used when generating the reference curves. This current value then identifies the glucose value stored in the LUT associated with the reference curve which had the same current value at the same specified time in its current decay phase. The stored value from the LUT which matches a contemporaneous value will identify the associated blood plasma glucose level so measured. The specified time (t_m) may be between about 1 sec. and about 15 sec. Different reference curves or LUTs may be used according to the measured temperature of the sensing strip, as determined by the thermocouple described herein for example. The control unit (63) is arranged to control the second voltage unit (62) to apply a DC voltage sequentially to the second to fourth pairs of drive electrodes, and to measure the resulting current in turn.

The calculating unit contains such a LUT and is arranged to compare respective contemporaneously measured current (decaying) values separately from each of the second to fourth sample zones, with stored reference current values, to identify the closest match (or interpolate between the closest two matches) and to retrieve an associated blood plasma glucose value "BG_raw" from the LUT associated with that match. There may comprise as plurality of LUTs which may be respectively associated with reference curves generated for a common specified temperature of blood sample. The calculating unit may be arranged to select the appropriate LUT based on the measured temperature of the sampling strip at the time of the measurement at hand.

The calculating unit is arranged to produce an adjusted value "BG_corrected" for the blood plasma glucose level so retrieved according to:

^BG corrected = f(^BGraw > ^HCT) where f(BG_raw,HCT) is a predetermined corrective function of the measured haematocrit value

HCT for the sample in the first sample zone, and o\BG_raw which is an uncorrected blood plasma glucose value measured for the blood samples in any one of (or an average of several or all of) the second to fourth sample zones. The form of the function

the predetermined corrective function may be selected by the user. One example is of the form: f(BG_rcm , HCT) = BG_rm - [m x (HCT) + c] where m is a positive or negative constant and c is a positive or negative constant. These values may be evaluated by calibration against commercially available calibration blood samples containing known HCT and glucose levels. This functional form exploits the finding that errors in uncorrected glucose measurements are typically linear to a first approximation, as a function of HCT, and that so too is the corrective function. Of course, other more accurate corrective functional forms may be used such as would be apparent to the skilled person in this field.

Figure 7 schematically illustrates circuitry comprising an ASIC (application specific integrated circuit) (85) arranged for connection to a microcontroller (80) integrated circuit arranged for use within the sensing unit (60, Fig.4) in conjunction with the sensing plate of Figure 3 comprising four sampling zones. The circuit is arranged to apply an AC drive voltage signal (at a frequency of 50 KHz) to the first pair of electrode terminals (31 , 32) and to apply DC drive voltage signals to each of the second to fourth pairs of drive electrodes (37 to 42) of the sampling plate (30) and is responsive to the resulting current signals to measure the resulting currents received from the sampling plate. The functions of the control unit (63) are performed by the microcontroller (80).

In particular, a DC voltage signal is output by the microcontroller (80) via a digital-to-analogue converter (81) and input to a current amplifier (86) formed within the ASIC (85) the amplified output of which is input to a switch unit (87) controllable by the microcontroller to selectively connect the DC signal to one of three voltage output terminals of the sensing unit respectively connected to an anode terminal (38, 40, 42) of a respective one of the second to fourth pairs of drive electrodes of the sensing plate when the latter is connected to the former in use as shown in Figures 4 and 7. A modulated AC voltage signal (square-wave) is output by the microcontroller via a PWM unit (82, pulse-width modulation) which is received by a 50KHz sinewave filter unit (88) formed on the ASIC connected to the PWM unit and arranged to generate a sinewave voltage signal having an amplitude determined by the microcontroller sand a frequency of 50 KHz. The AC voltage output of the sinewave filter is connected to an input of an amplifier unit (89) for amplification thereby. The output of the amplifier unit is connected to a voltage output terminal of the sensing unit connected to an anode terminal (32) of the first pair of drive electrodes of the sensing plate when the latter is connected to the former in use as shown in Figures 4 and 7.

A thermo-couple (53) of the sampling plate/strip is electrically connected to a further terminal (90) of the semsing unit via which it is placed in electrical connection with an ASIC controller unit (92), via an amplifier (91) - each formed on the ASIC - for use on measuring the temperature of the sensing strip.

The cathodes (37, 39, 41 , 31) of each of the four pairs of drive electrodes of the sampling strip are connected in common, via the conductive strip (43), to an input terminal of the sensing unit. The ASIC comprises a sensor unit (900) arranged to detect the presence of an AC signal or a DC signal at the common conductive strip connected to the input terminal and to selectively connect the common conductive strip either one of two signal transmission lines as follows. The first signal transmission line comprises a high-speed amplifier (93) and a high-speed voltage amplifier (94) in series connection with the input terminal for amplifying the input signal and are in subsequent series connection with a peak-detector unit (95) arranged to detect an AC amplitude in a received electrical current (therefrom to measure an AC current amplitude value) and a subsequent sample-and-hold unit (96) for storing the detected peak values for subsequently outputting them to the microcontroller via an analogue-to-digital converter (ADC, 84) thereof. The sensor unit thus connects the sampling strip to this signal transmission line when an AC signal is detected and haematocrit measurements are intended.

Alternatively, the second signal transmission line is employed, which comprises a high-speed amplifier (930) and a high-speed voltage amplifier (940) in series connection with the input terminal for amplifying the input signal and subsequently outputting it to the microcontroller via an analogue-to- digital converter (ADC, 84) thereof. The sensor unit thus connects the sampling strip to this signal transmission line when a DC signal is detected and glucose measurements are intended.

The ASIC controller unit (92) controls the timing and coordination of the components formed upon the ASIC under the master control of the microcontroller via an interface (83) of the microcontroller with which the ASIC control unit is in communication. In this way, the required current and voltage values may be applied to, and received from, the sampling plate (30) via the ASIC to enable the microcontroller to perform the measurements of HCT and blood glucose as described above. Figure 8 illustrates an alternative form of sampling strip (30A) and sensing unit in which each of the cathodes of the first to fourth pairs of drive electrodes has a separate output terminal on the sampling strip (30A) and the sensing unit has a corresponding separate respective four input terminals. This is as an alternative to the arrangements of Figure 3, 4 and 7 in which the sampling strip has one common terminal (43) for connecting to one input terminal of the sensing unit for receiving signals from each of the first to fourth sampling zones separately in time (e.g. sequentially).

In this example, the switch unit (87) is replaced with a multiplexer unit (99) for applying the DC signal from the microcontroller (80) simultaneously to each of the second to fourth drive electrode pairs. Three extra pairs of high-speed amplifiers (100, 101) which serve the same function as those amplifiers (93, 94) on the ASIC of Figure 7, are also provided to separately amplify signals received from the second to fourth drive electrode pairs via the three associated input terminals, respectively, of the sensing unit in connection with the cathodes of those drive electrode pairs. These separately, and simultaneously, amplify the DC current signals they receive. The amplified input signals (AC and DC) are passed from the ASIC t the microcontroller via ADC units (84A, 84B). In this way, the required current and voltage values may be applied simultaneously to, and received simultaneously from, the sampling plate (30) via the ASIC to enable the microcontroller to perform the measurements of HCT and blood glucose as described above.

Figure 9 schematically illustrates a further alternative form of sensing unit similar to the sensing unit if Figure 8, but arranged for use with the sensing strip (30B) of the type shown in Figure 3. Like elements are assigned like reference symbols.

Figure 9 shows an alternative form of ASIC (98B) arranged for connection to the microcontroller (80) integrated circuit arranged for use within the sensing unit (60, Fig.4) in conjunction with the sensing plate of Figure 3 comprising four sampling zones. The circuit is arranged to apply an AC drive voltage signal (at a frequency of 50 KHz) to the first pair of electrode terminals (31 , 32) for measuring haematocrit as described above, and to apply DC drive voltage signals to each of the second to fourth pairs of drive electrodes (37 to 42) of the sampling plate (30B). The ASIC is responsive to the resulting current signals to measure the resulting currents received from the sampling plate. The functions of the control unit (63) are performed by the microcontroller (80).

A modulated AC voltage signal (square-wave) is output by the microcontroller via a PWM unit (82, pulse-width modulation) which is received by a 50KHz sinewave filter unit (88) formed on the ASIC connected to the PWM unit and arranged to generate from the square waves received by it, a sinewave voltage signal having an amplitude determined by the microcontroller sand a frequency of 50 KHz. The AC voltage output of the sinewave filter is connected to an input of an amplifier unit (89) for amplification thereby. In particular, the AC voltage signal electrically connect to an input port (93A) of a first operational amplifier unit (93) formed in the ASIC. This operational amplifier has a second input port (93B) which serves as an input terminals of the sensing unit connected to an anode terminal (31 ) of the first pair of drive electrodes of the sensing plate when the latter is connected to the former in use as shown in Figures 4 and 9.

The first operational amplifier has an output port (93C) which is connected to its second input port via a feed-back loop comprising a resistor (103). As a result, the AC drive voltage (50KHz) applied to the first input port of this operational amplifier by the sinewave filter unit (88) is expressed as a correspondingly AC voltage level at the second input port (93B) of the operational amplifier. This produces an AC potential difference across the drive electrodes of the first electrode pair relative to a reference voltage (V_ref) provided by the ASIC (typically close to Earth value). Consequential conduction through a blood sample in the first sampling zone of the sampling strip enters this operational amplifier, being electrically connected to that particular blood sample. The result is that a measurable current is received by the operational amplifier at its second input port (93B).

The second input port of the first operational amplifier unit is connected to a voltage output terminal of the sensing unit connected to an anode terminal (31 ) of the first pair of drive electrodes of the sensing plate when the latter is connected to the former in use as shown in Figures 4 and 9. A DC voltage signal is output by the microcontroller (80) via a digital-to-analogue converter (81 ) and input to a current amplifier (86) formed within the ASIC the amplified output of which is input to a multiplexer unit (99) controllable by the microcontroller to output the DC signal to each one of three voltage output ports (102) which each electrically connect to an input port (100A) of a respective one of three further operational amplifier units (100) formed in the ASIC. Each of the three further operational amplifiers has a second input port (100B) which serves as a respective one of three input terminals of the sensing unit respectively connected to an anode terminal (38, 40, 42) of a respective one of the second to fourth pairs of drive electrodes of the sensing plate when the latter is connected to the former in use as shown in Figures 4 and 9.

Each of these three further operational amplifiers has a respective output port (100C) which is connected to its second input port via a feed-back loop comprising a resistor (103). As a result, the DC drive voltage (V_driVe) applied to the first input port of the respective further operational amplifier by the multiplexer unit (99) is expressed as a correspondingly substantially constant voltage level at the second input port (100B) of each of the three further operational amplifiers. This produces a controllably constant potential difference across the drive electrodes of the second to fourth electrode pairs relative to a reference voltage (V_ref) provided by the ASIC (typically close to Earth value). Consequential conduction through a blood sample in an associated one of the second to fourth sampling zones of the sampling strip enters the further operational amplifier electrically connected to that particular blood sample. The result is that a measurable current is received by the further operational amplifier at its second input port (100B). A thermo-couple (53) is arranged in the sensing unit and placed in electrical connection with an ASIC controller unit (92), via an amplifier (91 ) - each formed on the ASIC - for use on measuring the temperature of the sensing strip. The thermo-couple is arranged to physically contact the sensing strip when connected to the sampling unit in use thereby to measure the temperature of the sampling plate/strip.

The cathodes (37, 39, 41 , 31 ) of each of the four pairs of drive electrodes of the sampling strip are connected in common, via the conductive strip (43), to an input terminal of the sensing unit. The ASIC provides the reference voltage (V_ref) to that input terminal.

The ASIC controller unit (92) controls the timing and coordination of the components formed upon the ASIC under the master control of the microcontroller via an interface (83) of the microcontroller with which the ASIC control unit is in communication . In this way, the required current and voltage values may be applied to, and received from, the sampling plate (30B) via the ASIC to enable the microcontroller to perform the measurements of HCT and blood glucose as described above.

The above embodiments are intended to provide illustrative examples and are not intended to be limiting. Modifications to, and variants of the embodiments such as would be apparent to the skilled person are encompassed within the invention such as is defined , for example, by the claims.

Claims

CLAIMS:

A sampling plate for use in performing electrical measurements on a liquid sample containing blood comprising: a first sample zone for receiving a first liquid sample; a first pair of drive electrodes with separate respective electrode terminals spaced by a spacing for receiving a said first liquid sample within the first sample zone for use in driving an electrical current through the first sample; at least one further sample zone for receiving a further said liquid sample and containing a reagent to react with free glucose in the liquid sample; a further pair of drive electrodes with separate respective electrode terminals spaced by a second spacing for receiving a said further liquid sample within each said further sample zone for use in driving an electrical current through the further sample, wherein the first spacing is less than the second spacing.

A sampling plate according to any preceding claim in which the two drive electrode terminals of one some or each said pair of drive electrodes present to each other across a respective said spacing opposing electrode sides extending along the sample zone which define between them a drive gap for receiving a said sample therein whereby the drive electrodes are mounted upon a non-conducting substrate within the respective sample zone and are adapted for driving an electrical signal transversely across the drive gap through a said sample when supported upon the substrate.

A sampling plate according to any preceding claim wherein drive electrode terminals of said further pair(s) of drive electrodes present to each other across a respective said spacing opposing electrode sides extending along the sample zone which define between them a drive gap for receiving a said sample therein, wherein opposing said sides in each said further pair of drive electrodes are of unequal length.

A sampling plate according to any preceding claim in which said first spacing is substantially uniform along at least a part of its length.

5. A sampling plate according to any preceding claim in which each said second spacing is substantially uniform along at least a part of its length.

6. A sampling plate according to any preceding claim in which each of the drive electrodes is in electrical communication with a respective electrical contact zone provided on the sampling plate which is exposed for electrical connection simultaneously with an external drive voltage source or with an external sensing circuitry, respectively.

7. A sampling plate according to any preceding claim in which the first spacing is greater than about 80 microns and less than about 300 microns.

8. A sampling plate according to any preceding claim in which a said further spacing is greater than about 300 microns.

9. A sampling plate according to any preceding claim including two or more separate said further sample zones each for receiving a respective liquid sample and each containing a respective reagent to react with free glucose in the liquid sample; and in each said further sample zone a respective pair of drive electrodes with separate respective electrode terminals spaced by a spacing for receiving a said respective liquid sample within the further sample zone for use in driving an electrical current through the sample.

10. A sampling plate according to claim 9 wherein the at least two of, or each of, the said sample zones other than the first sample zone are substantially the same in shape and size.

1 1 . A sampling plate according to any of claims 9 to 10 wherein the at least two of, or each of, the said pairs of electrodes within respective said sample zones other than the first sample zone are substantially the same in shape and size.

12. A sampling plate according to any preceding claim in which a drive electrode of some or each of said pairs of drive electrodes is a single electrode common thereto.

13. A sampling apparatus for use in performing electrical measurements on a liquid sample containing blood, the apparatus comprising: a first output terminal arranged for outputting an alternating (AC) electrical voltage; and a second output terminal arranged outputting a direct (DC) electrical voltage applied thereto; and at least one input terminal for receiving an input electrical signal externally input thereto; and a control unit arranged to apply an alternating electrical voltage to the first output terminal and concurrently to measure a first electrical current at the at least one input terminal resulting therefrom when a said liquid sample is in electrical series connection between the first output terminal and the input terminal, and arranged to apply a direct electrical voltage to the second output terminal and concurrently to measure a second an electrical current at the at least one input terminal resulting therefrom when a liquid sample is in electrical series connection between the second output terminal and an input terminal; and a calculating unit arranged to calculate an electrical resistance value using a value of the first electrical voltage and a value of the concurrently measured first electrical current, and arranged to calculate a first calculated value representing the relative volume of red blood cells in the liquid sample according to the calculated electrical resistance value; and to calculate a second calculated value representing an amount of glucose in the liquid sample according to both the first calculated value and the measured second electrical current, and to output the result.

14. A sampling apparatus according to claim 13 in which the control unit is arranged to apply the alternating electrical voltage and the direct voltage concurrently.

15. A sampling apparatus according to any of claims 13 to 14 in which the calculating unit is arranged to produce according to the measured second current a stored value representing an amount of glucose in the sample which is un-associated with red blood cells and said second calculated value comprises the product of the first calculated value and the stored value.

16. A sampling apparatus according to any of claims 13 to 15 including a memory unit arranged to retrievably store a plurality of said stored values each in association with a respective said measured second current, wherein the calculating unit is arranged to produce a given said stored value by retrieving it from the memory unit according to said measured second current value with which it is associated.

17. A sampling apparatus according to any of claims 13 to 16 including a plurality of said second output terminals wherein said control unit is arranged for applying thereto a respective direct (DC) electrical voltage and concurrently to measure a respective said second electrical current resulting therefrom when a respective said liquid sample is in electrical series connection between the respective second output terminal and an input terminal; wherein the calculating unit is arranged to calculate said second calculated value representing an average of the amount of glucose in a plurality of said liquid samples according to both the first calculated value and the plurality of measured second electrical currents, and to output the result.

18. A sampling apparatus according to any of claims 13 to 17 including a socket adapted for receiving electrode terminals of a sampling plate wherein the first output terminal, the second output terminal and the at least one input terminal(s) are exposed within the socket for forming simultaneously an electrical contact with a plurality of the electrode terminals of a said sampling plate when received in the socket.

19. A sampling apparatus according to any of claims 13 to 18 including the sampling plate according to any of claims 1 to 12 in which one drive electrode of said first pair of drive electrodes is adapted to disconnectably connect electrically to said first output terminal and the other drive electrode of the first pair of drive electrodes is adapted to disconnectably connect concurrently to a said input terminal, and in which one drive electrode of said second pair of drive electrodes is adapted to disconnectably connect electrically to said second output terminal and the other drive electrode of the second pair of drive electrodes is adapted to diconnectably connect concurrently to a said input terminal, thereby to connect the first pair of drive electrodes and the second pair of drive electrodes to the sampling apparatus simultaneously for electrical communication therewith.

20. A sampling apparatus for use in performing electrical measurements on a liquid sample containing blood, the apparatus comprising: an output terminal for outputting an alternating electrical voltage (AC) applied thereto; an input terminal for receiving an input electrical signal externally input thereto; a control unit arranged to apply a said alternating electrical voltage signal and concurrently to measure an electrical current received at the input terminal resulting therefrom when a said liquid sample is in electrical series connection between the output terminal and the input terminal; a calculating unit arranged to calculate an electrical resistance value using said electrical voltage and said concurrently measured electrical current, and to calculate a value representing the relative volume of red blood cells in the liquid sample according to the calculated electrical resistance value, and to output the result.

A sampling apparatus according to any of claims 13 to 20 in which the calculating unit is arranged to calculate said first calculated value (HCT) according to the electrical resistance value (R) using the following equation:

22. A sampling apparatus according to any of claims 13 to 21 in which the frequency of said alternating electrical signal has a value in the range 1 KHz to 150KHz. 23. A sampling apparatus according to any of claims 13 to 22 in which the frequency of said alternating electrical signal has a value in the range 25KHz to 125KHz.

24. A sampling apparatus according to any of claims 13 to 23 in which the frequency of said alternating electrical signal has a value in the range 35KHz to 100KHz.

25. A sampling apparatus according to any of claims 13 to 24 in which the frequency of said alternating electrical signal has a value in the range 45KHz to 75KHz.

26. A sampling apparatus according to any of claims 13 to 25 in which the frequency of said alternating electrical signal has a value in the range 47KHz to 53KHz.

27. A sampling apparatus according to any of claims 13 to 26 in which said alternating electrical voltage has an amplitude in the range from about 0.05 volts to about 2 volts. 28. A sampling apparatus according to any of claims 13 to 27 in which said direct (DC) voltage is a potential difference between the second output terminal and a said at least one input terminal.

29. A sampling apparatus according to any of claims 13 to 28 in which said direct (DC) voltage is substantially constant in value.

30. A sampling apparatus according to any of claims 13 to 29 in which said alternating (AC) voltage is an alternating potential difference between the first output terminal and a said at least one input terminal.

31 . A sample measurement method for performing electrical measurements on a liquid sample containing blood, the method comprising: receiving the liquid sample on a sample plate comprising first electrode terminals separated by a first spacing adapted to be bridged by blood from the liquid sample; and applying to the electrodes an alternating electrical voltage having a given signal frequency to generate a first alternating potential difference across the first spacing; measuring a first electrical current passing across the first spacing and therefrom determining a value of an electrical resistance of the liquid sample bridging said first spacing, and determining a relative volume of red blood cells in the liquid sample using said resistance value; and receiving the liquid sample on the sample plate comprising second electrode terminals separated by a second spacing adapted to be bridged by blood from the liquid sample; and applying to the electrodes a direct (DC) voltage to generate a second direct potential difference across the second spacing; measuring a second electrical current passing across the second spacing; and, determining an amount of glucose in the liquid sample bridging said second spacing using the measured value of the second electrical current and the value of the relative volume of red blood cells in the liquid sample.

A method according to claim 31 in which said direct (DC) voltage is substantially constant in value.

A sample measurement method for performing electrical measurements on a liquid sample containing blood, the method comprising: receiving the liquid sample on a sample plate comprising electrode terminals separated by a spacing adapted to be bridged by blood from the liquid sample; and applying to the electrodes an alternating electrical voltage having a given signal frequency to generate an alternating potential difference across the spacing; measuring an electrical current passing across the spacing and therefrom determining a value of an electrical resistance of the liquid sample bridging said spacing; and, determining a relative volume of red blood cells in the liquid sample using said resistance value.

A sample measurement method according to any of claims 30 to 33 in which the frequency of said alternating electrical signal has a value in the range 1 KHz to 150KHz.

A sample measurement method according to any of claims 30 to 34 in which the frequency of said alternating electrical signal has a value in the range 25KHz to 125KHz.

36. A sample measurement method according to any of claims 30 to 35 in which the frequency of said alternating electrical signal has a value in the range 35KHz to 100KHz.

37. A sample measurement method according to any of claims 30 to 36 in which the frequency of said alternating electrical signal has a value in the range 45KHz to 75KHz.

38. A sample measurement method according to any of claims 30 to 37 in which the frequency of said alternating electrical signal has a value in the range 47KHz to 53KHz.