GB2402473A

GB2402473A - Analyte assay reading device involving sample flow rate measurement

Info

Publication number: GB2402473A
Application number: GB0312801A
Authority: GB
Inventors: Stephen Paul Sharrock
Original assignee: Inverness Medical Switzerland GmbH
Current assignee: Alere Switzerland GmbH
Priority date: 2003-06-04
Filing date: 2003-06-04
Publication date: 2004-12-08
Also published as: GB0312801D0

Abstract

An assay result reading device measures analyte concentration optically using LEDs 10, photodiodes 12 and a lateral flow assay test-strip having a plurality of zones. In addition to the measurement of the analyte, the rate at which a liquid sample moves along the assay test stick (between the zones) is measured and the result of the assay declared invalid if flow rate is outside predetermined acceptable limits. Flow rate may be measured optically using the same optical system as that used for the analyte detection. Also disclosed is the provision of different combinations of optical sources and detectors. The embodiment shows the use of three LEDs and two detectors, the detectors being common to more than one zone of the test strip.

Description

1 2402473 Title: Assay Result Reading Device and Method

Field of the Invention

This invention relates to devices for reading the results of assays for the quantitative or qualitative measurement of analyses, and to methods of using such devices. In particular it relates to diagnostic devices for the measurement of analyses in bodily fluids suitable for use in the home or in a point of care setting.

Background to the Invention

Analytical devices suitable for home testing of analyses are now widely commercially available. An immunoassay device suitable for this purpose for the measurement of the pregnancy hormone human chorionic gonadotropin (hCG) is sold by Unipath under the brand-name ClearBlue RIM and is disclosed in EP291194.

EP291194 discloses an immunoassay device comprising a porous carrier containing: a particulate labelled specific binding reagent for an analyte, which reagent is freely mobile when in the moist state; and an unlabelled specific binding reagent for the same analyte, which reagent is immobilised in a detection zone or test zone downstream from the unlabelled specific binding reagent. Liquid sample suspected of containing analyte is applied to the porous carrier whereupon it interacts with the particulate labelled binding reagent to form an analyte-binding partner complex. The particulate label is coloured and is typically gold or a dyed polymer, for example latex or polyurethane. The complex thereafter migrates into a detection zone whereupon it forms a further complex with the immobilised unlabelled specific binding reagent enabling the extent of analyte present to be detected or observed. Due to the nature of the binding reactions taking place it is necessary to wait for a particular period of time to elapse after the test has commenced in order to read the result. This is particularity important for a visual, semi-quantitative type of test whereby the detection zone or read line develops over time.

Various methods of timing the result have been proposed for commercial devices, including instructions to the user wait for a particular length of time before reading the assay result. Other methods include a signal that is generated after a particular period of time has elapsed, as disclosed in our copending application no. PCT/EP03/00274 which signal informs the user that the assay result should now be read.

As a control and to ensure the correct functioning of the device, a control zone is generally provided downstream from the measurement zone. A third binding reagent, which is able to bind with the first labelled reagent, is immobilized at this control zone such that in the absence of analyte, the user will be able to check if the test has been carried out correctly.

EP653625 discloses a lateral flow assay test-strip for use in combination with an assay reader whereby the extent of binding of particulate label is determined optically. It is also known from US5580794 to provide an integrated assay device and lateral flow assay test- strip wherein the result is determined optically using reflectance measurements.

US5837546 discloses a method of automatically starting an immunoassay device whereby a lateral flow carrier is provided with additional electrodes which sense the presence of fluid on the test-strip and a signal is generated which switches on the sensing electronics.

Due to the nature of a lateral flow type test which requires the release of a labelled particulate binding reagent, flow of liquid along a carrier (typically porous) and capture of the analyte complex in the detection zone, it is desirable to optimise the properties of the porous carrier.

The pore size of the carrier is an important consideration and is preferably chosen to be between 1-12 m. The carrier is conveniently nitrocellulose, the pore size of which may vary in part due to the manufacturing process. The assay device may additionally have a wick in fluid communication with the carrier which serves to collect the liquid sample and the carrier typically comprises two pieces of different materials. Nitrocellulose is typically used as the carrier material for the assay strip and has considerable advantages over conventional strip materials, such as paper, because of its natural ability to bind proteins without requiring prior sensitization. In order to optimise the assay, the nitrocellulose is typically subjected prior to use to a number of treatments which include the use of blocking agents such polyvinylalcohol and the useof soluble glazes such as sugar to enhance release of the labelled reagent.

The present inventors have observed that the flow rate of fluid along the porous carrier may vary from test to test. In some cases the carrier has a tendency to flood, i.e. the fluid front moves along the carrier at a faster rate than normal. Conversely, in some cases, it has been noted that the fluid front moves along the carrier at a much slower rate than normal, namely the carrier is blocked to some extent. It has been found that these different types of fluid flow-rate behaviour can give rise to inaccurate results.

Due to the inconsistent nature of the materials used for both the wick and the porous membrane, the optimum point in time (after application of the liquid sample) for reading the result can be variable.

In the interests of providing devices which are inherently more accurate and reliable, it would be desirable to provide alternative or additional control features which would be able to determine the extent and/or rate at which the liquid sample moved along the porous carrier and to reject those readings where the flow rate was determined to fall outside of predetermined limits.

It would also be desirable to provide a method wherein the optimum time for reading the result could be reliably and reproducibly determined.

Summary of the Invention

It is an objective of the invention to provide an assay device comprising a reader for use in conjunction with a lateral flow test-strip which is able to optically measure analyte concentrations quantitatively and/or qualitatively with a high degree of reliability and accuracy.

It is a further objective of the invention to provide an assay reader, especially one for use in conjunction with a lateral flow test-strip, as well as a method of performing an analyte measurement, wherein the extent and/or rate of fluid flow along the test strip may be determined and wherein the final assay result may be rejected where the fluid flow rate has been determined to fall outside of certain predetermined limits.

One or both of the aforementioned objects may be met by the present invention, having the features defined and described below.

In a first aspect the invention provides an assay result reading device for reading the result of an assay performed using a liquid transport means; the reading device comprising means for calculating the rate and/or extent of the progress of a liquid sample applied or introduced to the liquid transport means; and means for determining whether the detected rate and/or extent of the progress of the liquid sample along the liquid transport means is within predetermined acceptable limits. If the rate of progress for example is above a predetermined acceptable limit, this is indicative that the liquid transport means (typically a porous carrier) may have become flooded. Conversely, if the rate of progress is below an acceptable limit, this may be indicative that, for example, the pores of the porous carrier may have become blocked. Conveniently therefore the predetermined acceptable limits will comprise an upper limit and a lower limit, with an acceptable range defined therebetween.

In a second aspect the invention provides a method of performing an assay for an analyte of interest in a liquid sample, the method comprising the steps of: applying or introducing the liquid sample to a liquid transport means, the liquid transport means being inserted into, or forming part of, an assay result reading device; calculating the rate and/or extent of the progress of the liquid sample along the liquid transport means; and determining whether the detected rate and/or extent of the progress of the liquid sample is within predetermined acceptable limits. Conveniently the method of the second aspect involves the use of an assay result reading device in accordance with the first aspect of the invention.

The liquid transport means preferably comprises a porous carrier, such as a lateral flow assay test strip of the sort which are well known to those skilled in the art. Alternatively the liquid transport means may comprise a capillary fill chamber, channel or the like (e.g. as disclosed in US 6 113 885). The liquid transport means may be an integral part of the assay result reading device e.g. in the manner disclosed in US 5 580 794. In such an embodiment the combined reading device/liquid transport means would typically be disposable. Alternatively the liquid transport means may be a separate component, which is normally introduced into the assay result reading device during the course of performing an assay. In this latter embodiment, the liquid transport means (typically a lateral flow assay test strip) would generally be cheap and disposable after a single use, whilst the assay result reading device would be re-usable and relatively expensive.

In a third aspect, the invention explicitly encompasses the combination of an assay result reading device in accordance with the first aspect of the invention together with a liquid transport means.

In describing the various aspects of the present invention, a number of terms are defined as follows: "Fluid sample" refers to any liquid material suspected of containing the analyte of interest.

Such samples may include human, animal or man-made samples. Typically, the sample is an aqueous solution or biological fluid.

Examples of biological fluids include urine, blood, serum, plasma, saliva, interstitial fluid and so on. Other samples which can be used include water, food products, soil extracts and the like for the performance of industrial, environmental, or food production assays as well as medical diagnostic assays. In addition, a solid material suspected of containing the analyte can be used as the test sample once it is modified to form a liquid medium which may include further treatment in order to release the analyte.

Any suitable analyte or analyses of interest may be measured.- Analytes that are particularly of interest include proteins, haptens, immunoglobulins, hormones, polynucleotides, steroids, drugs, infectious disease agents (e. g. of bacterial or viral origin) such as Streptococcus, Neisseria and Chlamydia, drugs of abuse, and biological markers such as cardiac markers and so on.

Typically the assay result reading device and the method of the invention are adapted to perform a diagnostic assay i.e. to provide information about the health status of a mammalian (typically a human) individual subject.

It is generally preferred, in all the aspects of the invention, to calculate the rate of progress of the liquid sample (rather than the extent thereof) along the liquid transport means.

Conveniently the flow rate is calculated between two zones on the liquid transport means, such that the presence of the liquid sample at, or passage thereof through, a first, upstream zone is detected, and likewise the present of the liquid sample at, or passage through, a second, downstream zone is detected. If the distance between the two zones is fixed and/or known, the relative or absolute flow rate of the liquid sample can be readily calculated by measuring the amount of time which elapses between detection of the liquid sample at the first and second zones.

In principle, the first and second zones may be anywhere on the liquid transport means so, for example, the first zone could be at the extreme upstream end and the second zone could at the extreme downstream end. The distance between the two zones (and therefore the time travel of fluid sample) may be chosen to be any that is convenient and is likely to depend upon the nature of the analyte to be determined and the physical dimensions and characteristics of the liquid transport means. For example the liquid transport means may comprise one or more microfluidic channels optionally containing one or more various microfluidic elements such as a red-blood cell separation means, time-gates, or fluid rate controlling means, all of which will influence the rate of travel of sample. In practice, it is desirable that the two zones are at a separation such that, at normal flow rates, a sufficently accurate flow rate may be calculated within the time frame of the assay, so as not to delay the assay process or assay result determination. For an assay for the detection and/or quantification of the pregnancy hormone hCG, for example, a desirable time would be between 5 and 60 seconds.

Advantageously the presence of the liquid sample at, or passage through, one or more additional zones on the liquid transport means is detected. This allows for a more accurate calculation of the flow rate. A larger number of flow rate calculation zones may be advantageous when the acceptable range of flow rates is rather narrow, or where the flow rate may vary at different portions of the liquid transport means (e.g. where there are portions with different flow characteristics, for instance, due to the incorporation of microfluidic elements).

In addition the provision of a plurality of "check zones" allows for checking that the liquid sample progresses through each of the zones in the expected sequence, thereby alerting the user to an abnormal flow pattern if the liquid sample is detected at a downstream zone in advance of detection at a particular upstream zone. Such abnormal flow patterns can occur for instance when a porous carrier is flooded by liquid samples ("oversampling").

If the calculated flow rate is outside of the predetermined acceptable limits, then the result of the assay may be declared invalid. Thus the flow rate calculation can act as a control feature. If the calculated flow rate is too high, due to flooding of the porous carrier (e.g. as a result of oversampling; or a result of a faulty assay device due to defects in manufacture, or damage in storage or in use) the user can be alerted and the assay result disregarded. Equally, if the calculated flow rate is too low (e.g. due to undersampling) the assay result can be disregarded. Thus the invention provides a means of avoiding errors due to, for example, over- or undersampling.

In principle any property of the liquid sample could be measured in order to calculate the rate and/or extent of progress of the liquid, such as its electrical capacitance, conductivity or resistivity. The porous carrier or other liquid transport means may comprise a substance which undergoes a detectable change in the presence of the liquid sample. For example nitrocellulose, commonly used as a porous carrier in lateral flow assay strips, is opaque (or substantially so) when dry, but its opacity is significantly reduced upon wetting.

Thus measurement or detection of the change in optical reflectance or transmissivity of a nitrocellulose carrier upon wetting by a liquid sample may be sufficient to detect the rate and/or extent of progress of the liquid sample.

Preferably the means for calculating the rate and/or extent of the progress of the liquid sample applied to the liquid transport means comprises an optical detection system. Such an optical detection system will typically generate one or more signals (advantageously, electrical signals) in a manner responsive to the rate and/or extent of the progress of the liquid sample. In a preferred embodiment, a suitable optical system comprises at least two light sources and at least one photodetector, or conversely at least one light source and at least two photodetectors, so as to be able to make optical measurements at at least two spatially separated zones of the liquid transport means.

In principle the light source could be external to the assay result reader e.g. ambient light.

However, this is extremely likely to introduce variation, and it is therefore greatly preferred that: (a) the assay result reading device is provided with at least one integral light source (LEDs are found especially convenient in this regard); and (b) the assay result reading device is provided with a housing or casing which substantially excludes, or at least greatly restricts, ambient light from entering the interior of the reading device. For present purposes, a housing or casing will be considered to substantially exclude ambient light if less than 10%, preferably less than 5%, and most preferably less than 1%, of the visible light incident upon the exterior of the device penetrates to the interior of the device.

A light-impermeable synthetic plastics material such as polycarbonate, ABS, polystyrene, polystyrol, high density polyethylene, or polypropylene containing an appropriate light- blocking pigment is a suitable choice for use in fabrication of the housing. An aperture may be provided on the exterior of the housing which communicates with the interior space within the housing: a test strip or similar porous carrier may be inserted through the aperture so as to perform an assay.

The liquid sample per se may have an optical property (e.g. colour) which renders it amenable to optical detection and/or monitoring of its progress along the liquid transport means. For example, a blood sample will absorb strongly in the range 400nm to 600nm, due to the presence of haemoglobin. Alternatively the liquid sample may be doped, prior to application to the liquid transport means, with a readily detectable substance (e.g. a dye, fluorochrome or the like) which will not interfere with the performance of the assay but will facilitate detection (especially optical detection) of the rate and/or extent of the progress of the liquid sample.

In yet another arrangement, the liquid transport means is provided with a readily detectable substance which is transported by the liquid sample. Again, a dye, fluorochrome or the like may be suitable in this regard. The readily detectable substance may conveniently be releasably immobilised on a porous carrier or the like, so as to be released upon contact with the liquid sample. The readily detectable substance may be e. g. a coloured substance which does not interfere with the assay. In a preferred embodiment the readily detectable substance is a particulate label which is attached to a mobilisable specific binding reagent (having specific binding for the analyte), and detection of which label in a detection zone constitutes an essential feature of the assay.

The particulate label may be anything suitable for the purpose, including coloured latex, a dye sol, or particulate gold. Alternatively the particulate label may comprise a fluorophore which can be excited by an LED emitting radiation of a suitable wavelength.

The preferred optical detection system will comprise at least one light source and at least one photodetector (such as a photodiode). Preferred light sources are light emitting diodes or LEDs. Reflected light and/or transmitted light may be measured by the photodetector.

For the purposes of this invention, reflected light is taken to mean that light from the light source is reflected from the porous carrier or other liquid transport means onto the photodetector. In this situation, the detector is typically provided on the same side of the carrier as the light source. Transmitted light refers to light that passes through the carrier and typically the detector is provided on the opposite side of the carrier to the light source.

For the purposes of a reflectance measurement, the carrier may be provided with a backing such as a white reflective MylarRTM plastic layer. Thus light from the light source will fall upon the carrier, some will be reflected from its surface and some will penetrate into the carrier and be reflected at any depth up to and including the depth at which the reflective layer is provided. Thus, a reflectance type of measurement may actually involve transmission of light through at least some of the thickness of the porous carrier.

In one embodiment the reader comprises a housing in which is contained at least two light sources (e.g. LEDs) and respective photodetectors arranged to receive light from the LEDs.

One of the light sources illuminates a first, upstream zone of the liquid transport means and another light source illuminates a second, downstream zone of the liquid transport means, and respective photodetectors are provided to detected light reflected and/or transmitted from the respective zones, the amount of such light which is reflected and/or transmitted depending on whether the liquid sample (optionally together with any light- absorbing or light-emitting substance transported thereby) has reached the zone(s) in question.

In a particularly preferred embodiment the assay result reading device comprises three light sources which illuminate respective first, second and third zones of the liquid transport means, and the flow rate of the liquid sample between at least two of the zones is measured.

Conveniently one of the zones from which measurements are made in the calculation of the flow rate is also a zone from which measurements are made in determining the result of the assay, for example, the first zone may be a zone in which analyte-specific labelled binding reagent is immobilised if analyte is present in the sample. Such a zone may be referred to as a test zone.

_

Desirably one of the zones from which measurements are made in the calculation of the flow rate is also a zone from which control measurements are made for the purpose of obtaining a control value, which is used to determine if the assay has been correctly performed. Such a zone may be referred to as a control zone.

It is advantageous that there is a zone, from which measurements are made in the calculation of the flow rate, which is also a zone from which measurements are made in calibration of the assay result reader. Such a zone may be referred to as a reference zone.

It is desirable that the components of the assay reader used in the detection and/or quantification of the analyte of interest are also used in calculating the flow rate of the liquid. This confers advantages of simplicity and economy, which are especially desirable for a disposable device. In particular, a preferred assay result reader has an optical detection system for detecting the presence and/or amount of the analyte of interest, and the same optical detection system is employed to make measurements for the purpose of calculating flow rates.

In a particularly preferred embodiment the assay result reader obtains measurements from a control zone, a reference zone and a test zone, the control zone being downstream from the reference zone which is itself downstream of the test zone (i.e. the reference zone is between the test and control zones). The reference zone allows for, inter alla, measurement of an optical property (e.g. reflectance and/or transmissivity) of the liquid transport means when wetted (e.g. a wetted porous carrier). Conveniently results obtained from the test and control zones are normalised relative to the reference zone, and this takes into account and compensates for any variation in the optical property of the sample. This is especially important when using biological samples, such as urine, which may vary widely in composition (e.g. concentration) and therefore vary in colour or colour intensity.

The housing of the assay result reader typically comprises an aperture such that a test strip may be releasably inserted into and (preferably) engaged with the housing. The housing is designed such that ambient light reaching the interior of the reading device is kept to an absolute minimum. Desirably suitable alignment and fixing means are provided within the housing such that the test strip remains in a fixed position when inserted. The light sources are arranged in the housing such that, when the test strip has been correctly inserted, the light sources are correctly aligned with the respective zones to be measured.

The assay test strip may be any conventional lateral flow assay test strip such as disclosed in EP291194 or US6352862. The test strip preferably comprises a porous carrier containing a particulate labelled specific binding reagent and an unlabelled specific binding reagent. The light sources and corresponding photodetectors are preferably so aligned such that during use, light from the light source or sources falls upon the respective zones on the porous carrier and is reflected or transmitted to the respective photodetectors. The photodetectors generate a current roughly proportional to the amount of light falling upon it which is then fed through a resistor to generate a voltage. The amount of light reaching the photodetector depends upon the amount of coloured particulate label present and therefore the amount of analyte. Thus the amount of analyte present in the sample may be determined. This method of optically determining the analyte concentration is described more fully in EP653625.

Alternatively, instead of using a test strip comprising a lateral flow porous carrier such as described by EP291194, a test strip having the binding reagents disposed within a capillary could be used, such as disclosed by US6113885.

In order to conduct an assay measurement using a assay result reading device in accordance with some of the preferred features of the invention, a test strip is inserted into the reader, and a liquid sample is then added to a sample receiving portion of the test strip.

Alternatively a liquid sample may be applied to the test strip first, and the strip then inserted into the reader. The sample migrates along the porous carrier and reaches a first zone, typically the test zone. When sample is added to the strip, a coloured particulate label is resuspended and migrates along the carrier along with the fluid. As the fluid front of the sample reaches first zone, there is a reduction in light intensity reaching the photodetector since the coloured particulate label absorbs some of the light. This change in reflected or transmitted light intensity is recorded. In practice, a larger amount of the particulate label is present in the initial fluid front than in the subsequent fluid. In addition, if a binding reaction takes place in the test zone due to the presence of analyte, particulate label will tend to remain in the test zone. Thus the shape of the resultant voltage - time profile observed will depend upon whether the zone is a test, control or reference zone.

For a three zone system, three voltage-time profiles will be recorded one for each zone, having a time lag due to the fact that measurement zones are spatially separated from one another and thus the time taken for the fluid front to reach the first zone is less than that taken to reach the second and so on.

From analysis of the voltage-time profiles for the respective zones and with knowledge of the distance between the zones, the rate of fluid flow may be determined. By use of a simple algorithm, the final assay reading may be rejected if the calculated flow rate has been determined to be too low or too high.

In a typical embodiment, the assay result reading device will typically further comprise one or more of the following: a central processing unit (CPU) or microcontroller; two or more LEDs; two or more photodiodes; a power source; and associated electrical circuitry. The power source may comprise a battery or any other suitable power source (e. g. a photovoltaic cell). The CPU will typically be programmed so as to determine whether the calculated rate and/or extent of progress of the liquid sample is within predetermined limits.

Conveniently the assay result reading device will comprise some manner of indicating the result of the assay to a user. This may take the form, forexample, of an audible or visible signal. Desirably the device will comprise a visual display to display the assay result.

This may simply take the form of one or more LEDs or other light sources, such that illumination of a particular light source or combination of light sources conveys the necessary information to the user. Alternatively the device may be provided with an alphanumeric or other display, such as an LCD. In addition, or as an alternative, to displaying the assay result, the device may also display or indicate in some other way to the user whether the calculated rate and/or extent of progress of the liquid sample is within the predetermined acceptable limits, and thus whether or not the result of the particular assay should be disregarded. If the reading device determines that a particular assay result should be disregarded it may prompt the user to repeat the assay. Displays suitable for displaying this sort of information are known to those skilled in the art and disclosed, for example, in WO 99/51989.

For the avoidance of doubt, it is expressly stated that any of the features of the invention described as "preferred" "desirable" "convenient", "advantageous" or the like may be adopted in combination with any other feature or features so-described, or may be adopted in isolation, unless the context dictates otherwise.

The invention will now be further described by way of illustrative example and with reference to the following drawings in which: Figure 1 is perspective view of one embodiment of an assay result reading device in accordance with the invention; Figure 2 is a block diagram illustrating schematically some of the internal components of the reading device embodiment depicted in Figure 1; and Figures 3-5 are graphs showing various signals returned from different portions of a test stick, inserted into the reading device illustrated in Figures 1 & 2, and their variation with time.

Example 1

An embodiment of an assay result reading device in accordance with the invention is illustrated in Figure 1.

The reading device is about 12cm long and about 2cm wide and is generally finger or cigar-shaped. The device comprises a housing 2 formed from a polystyrol light- impermeable synthetic plastics material, loaded with a carbon-based additive. At one end of the reading device is a narrow slot or aperture 4 by which an assay device (not shown) can be inserted into the reader.

On its upper face the reader comprises two oval-shaped apertures. One aperture accommodates the screen of a liquid crystal display 6 which displays information to a user e.g. the results of an assay, in qualitative or quantitative terms. The other aperture accommodates an eject mechanism 8 which, when actuated, forcibly ejects an inserted assay device from the assay result reading device.

The assay device for use with the reading device is a generally conventional lateral flow test stick e.g. of the sort disclosed in US 6, 156,271, US 5,504,013, EP 728309 or EP 782707. The assay device and a surface or surfaces of the slot in the reader, into which the assay device is inserted, are so shaped and dimensioned that (1) the assay device can only be successfully inserted into the reader in the appropriate orientation; and (2) there is a precise three dimensional alignment of the reader and an inserted assay device, which ensures that the assay result can be read correctly the reader.

A suitable assay device/reader device combination exhibiting this precise three dimensional alignment is disclosed in EP 833145.

When an assay device is correctly inserted into the reader, a switch is closed which activates the reader from a "dormant" mode, which is the normal state adopted by the reader, thereby reducing energy consumption.

Enclosed within the housing of the reader (and therefore not visible in Figure 1) are a number of further components, illustrated schematically in Figure 2.

Referring to Figure 2, the reader comprises three LEDs 10a,b, and c. When a test stick is inserted into the reader, each LED10 is aligned with a respective zone of the test stick.

LED 10a is aligned with the test zone, LED 10b is aligned with the reference zone and LED 10c is aligned with the control zone. Respective photodiodes 12 detect light reflected from the various zones and generate a current, the magnitude of which is proportional to the amount of light incident upon the photodiodes 12. The current is converted into a voltage, buffered by buffer 14 and fed into an analogue to digital converter (ADC, 16).

The resulting digital signal is read by microcontroller 18.

In a simple arrangement, a separate photodiode is provided to detect from each zone (i.e. the number of photodiodes equals the number of zones from which reflected light measurements are made). The arrangement illustrated in Figure 2 is more sophisticated, and preferred. Two photodiodes 12 are provided. One photodiode detects light reflected from the test zone and some of the light reflected from the reference zone. The other photodiode 12 detects some of the light reflected from the reference zone and the light reflected from the control zone. The microcontroller 18 switches on the LEDs 10 one at a time, so that only one of the three zones is illuminated at any given time - in this way the signals generated by light reflected from the respective zones can be discriminated on a temporal basis.

Figure 2 further shows, schematically, the switch 20 which is closed by insertion of an assay device into the reader, and which activates the microcontroller 18. Although not shown in Figure 2, the device further comprises a power source (typically one or two button cells), and an LCD device responsive to output from the microcontroller 18.

In use, a dry test stick (i.e. prior to contacting the sample) is inserted into the reader, this closes the switch 20 activating the reader device, which then performs an initial calibration. The intensity of light output from different LEDs is rarely identical. 17

Similarly, the respective photodetectors are unlikely to have identical sensitivities. Since such variation could affect the assay reading an initial calibration is effected, in which the microcontroller adjusts the length of time that each of the three LEDs is illuminated, so that the measured signal from each of the three zones (test, reference and control) is approximately equal and at a suitable operating position in a linear region of the response profile of the system (such that a change in intensity of light reflected from the various zones produces a directly proportional change in signal).

After performing the initial calibration, the device performs a further, finer calibration.

This involves taking a measurement ("calibration value") of reflected light intensity for each zone whilst the test stick is dry - subsequent measurements ("test values") are normalised by reference to the calibration value for the respective zones (i.e. normalised value = test value/calibration value).

To conduct an assay, a sample receiving portion of the test stick is contacted with the liquid sample. In this case of a urine sample, the sample receiving portion may be held in a urine stream, or a urine sample collected in a receptacle and the sample receiving portion briefly (for about 5-20 seconds) immersed in the sample. Sampling may be performed whilst the test stick is inserted in the reader or, less preferably, the stick can be briefly removed from the reader for sampling and then reintroduced into the reader.

Measurements of reflected light intensity from one or more (preferably all three) of the zones are then commenced, typically after a specific timed interval following insertion of the test stick into the reader. Desirably the measurements are taken at regular intervals (e.g. at between 1-10 second intervals, preferably at between 1-5 second intervals). The measurements are made as a sequence of many readings over short (10 milliseconds or less) periods of time, interleaved zone by zone, thereby minimising any effects due to variation of ambient light intensity which may penetrate into the interior of the reader housing.

Figure 3 is a graph showing the intensity of reflected light (arbitrary values) against time detected from each of the three zones, using a sample which does not contain the analyte of interest. The profile for the test zone is indicated by crosses, that for the reference zone by circles, and that for the control zone by triangles.

Considering the test zone profile, there is an initial lag phase during which the liquid sample is migrating along the porous carrier. In this period, the level of light reflected by the test zone is essentially constant. As the sample reached the test zone the amount of light reflected sharply decreases. This is primarily due to absorption of light by the coloured particulate label transported by the liquid sample. However some of the reduction in reflected light intensity is simply due to wetting of the nitrocellulose porous carrier, since dry nitrocellulose is more reflective.

As the fluid front moves past the test zone the level of reflected light starts to increase, the coloured label being transported with the sample downstream past the test zone. The reflected light intensity does not return to the original level because of the wetting of the mitrocellulose and because a small amount of the coloured particulate label is left behind as the liquid advances.

Generally similar profiles are exhibited by the reference and control zones, although these are downstream of the test zone and so lag further behind. The control zone profile, in particular, does not return to its original level of reflected light intensity because of development of the "control line" (i.e. deposition of coloured particulate label in the control zone).

Figure 4 is essentially similar, and shows the profiles obtained using % normalized results (i.e. test value divided by calibration value xlOO). The profile being expressed in terms of % of calibration value against time. Figure 4 demonstrates that normalization of the test readings against an initial calibration reading reduces the variation in signal from the test, reference and control zones (although again the control zone value remains low due to the deposition of labelled reagent in the control zone).

In order to calculate the flow rate of the liquid sample along the porous carrier, the exemplified reading device actually compares the normalised results from the test and control zones with the result obtained from the reference zone in order to arrive at a "Relative attenuation of reflected light intensity" (HA).

[ % A = (Ref(t)/Ref(cal) - Test(t)Test(cal) ] Ref(t)/Ref(cal) Typical MA profiles (against time), for a sample containing a relevant analyte of interest, are shown in Figure 5. A positive attenuation means that the zone in question is reflecting less light than the reference zone, whilst a negative attenuation means that the zone in question is reflecting more light than the reference zone.

Referring to the PA profile of the test zone, it is apparent that the test zone signal is initially greatly attenuated (relative to the reference zone) when the liquid sample (with coloured particulate label) reached the test zone, but has not yet reached the reference zone. After about 35 seconds, the liquid sample starts to reach the reference zone and this leads to a sudden drop in the relative attenuation of the test zone. After about 40 seconds, the fluid front starts to leave the reference zone leading to an increase in the reflectivity of the reference zone and therefore an increase in the relative attenuation of the test zone.

This levels off and eventually reaches a plateau, at a positive attenuation of just under 30%, the test zone having captured some of the coloured particulate label due to the presence in the sample of the analyte of interest.

Considering the profile for the control zone, it is apparent that there is an initial sharp fall (negative attenuation), since the liquid sample reaches the reference zone before the control zone. As the liquid sample starts to leave the reference zone before the control zone the relative negative attenuation in signal from the control zone starts to return to zero, and as the liquid samples reached the control zone the relative attenuation becomes positive and reached a plateau level of about 15%, due to the deposition of labelled reagent in the control zone to provide a positive control result.

Whilst in the presently exemplified reader the test zone result is compared with the reference zone result, a useful alternative would be to compare the test zone result with the control zone result.

In general terms the flow rate is calculated by detecting the change in reflected light intensity associated with the arrival of the liquid sample at a particular zone, and determining the time which elapses between the arrival of the liquid sample at the various zones. More precisely, the flow rate is calculated as described below.

The signal at all three zones is measured irrespective of the position of liquid on the test strip.

The signal attenuation at the test zone is measured with respect the signal attenuation at the reference zone. When the fluid front arrives at the test zone the signal attenuation will change relative to the reference zone, due to the fluid front not yet having reached the reference zone (it being positioned downstream from the test zone). Timing is commenced when the signal attenuation of the test zone relative to the reference zone is greater than 10%. It should be mentioned that the value of 10% indicates the degree of confidence including any margin of error which has been attached to the measurement reading, which in itself depends on the various measurement parameters, e.g. test strip, optics. This might vary and be chosen to be any convenient value.

The liquid then proceeds into the reference zone and when the signal attenuation of the control zone relative to the reference zone is greater than minus 10% (-10%), the device considers that the liquid has reached the control zone (the minus value reflecting that the control zone is positioned downstream from the test zone). When the signal attenuation of the control zone relative to the reference zone is greater (i.e. more positive) than zero, the device determines that the liquid has reached the control zone. Thus the time measurement by the device may not necessarily exactly correspond to the time when the fluid arrives at the respective zones.

Although in this example the reader measures the rate of passage of liquid between the test and control zones, it measures it with respect to the signal obtained from the reference zone. However the arrival of liquid at the test and control zones could be determined absolutely, (i.e. not by measurement with respect to the reference zone).

The reader is also programmed to declare an assay result invalid if the liquid sample is detected at the control zone before it is detected at the reference zone, as this is indicative that the liquid sample has followed an abnormal flow path.

Claims

Claims 1. An assay result reading device for reading the result of an

assay performed using a - -- - liquid transport means; the reading device comprising means for calculating the rate and/or extent of the progress of a liquid sample applied or introduced to the liquid transport means; and means for determining whether the detected rate and/or extent of the progress of the liquid sample along the liquid transport means is within predetermined acceptable limits.
2. An assay result reading device according to claim 1, wherein the rate and/or extent of the progress of the liquid sample is detected by measurement of an optical property of the liquid or of a substance transported by the liquid, or of the liquid transport means.
3. A device according to claim 1 or 2, wherein the liquid transport means comprises a porous carrier.
4. A device according to any one of the preceding claims comprising one or more integral light sources.
5. A device according to claim 4, wherein the one or more light sources comprise light emitting diodes.
6. A device according to any one of the preceding claims, comprising one or more photodetectors.
7. A device according to claim 6, wherein the one or more photodetectors comprise photodiodes.
8. A device according to any one of the preceding claims, wherein the rate and/or extent of the progress of the liquid sample is calculated by measurement of the optical reflectance and/or transmissivity of the porous carrier, which properties are detectably altered by passage and/or accumulation of a coloured particulate labelled binding reagent which is transported by the liquid sample.
9. A device according to any one of the preceding claims, comprising a microcontroller or central processing unit (CPU) which is suitably programmed so as to determine whether the calculated rate and/or extent of the progress of the liquid sample is within predetermined acceptable limits.
10. A method of performing an assay for an analyte of interest in a liquid sample, the method comprising the steps of: applying or introducing the liquid sample to a liquid transport means, the liquid transport means being inserted into, or forming part of, an assay result reading device; calculating the rate and/or extent of the progress of the liquid sample along the liquid transport means; and determining whether the detected rate and/or extent of the progress of the liquid sample is within predetermined acceptable limits.
11. A method according to claim 10, comprising use of an assay result reading device in accordance with any one of claims 1-9.
12. An assay result reading device substantially as hereinbefore described and with reference to the accompanying drawings.