ES2377681T3 - Measurement of substances in liquids - Google Patents

Abstract

A disposable test strip for measuring the concentration of a substance in a sample liquid, said device comprising: A first active sensor working area to generate load carriers in proportion to the concentration of said substance in the sample liquid; said first active working sensor area comprises an electrode (8b); a second active working sensor area, said second active working sensor area comprises an electrode (6b); and a reference sensor (4b) located upstream of said first and second active working sensing areas, whose reference sensor is a common reference for both the first and second active working sensing areas, characterized in that said second active area working sensor is located downstream of said first active working sensor area and generates load carriers in proportion to the concentration of said substance in the sample liquid, said first and second active working sensor areas are substantially identical, such that In the absence of an error condition, the amount of said load carriers generated by said first active work sensor area is substantially identical to the amount of said load carriers generated by said second active work sensor area and, said reference sensor and said first and second working sensor parts, arranged in such a way that the l sample liquid is constrained to flow substantially in one direction through said reference sensor and said first and second active areas sensing work.

Description

Measurement of substances in liquids.

The present invention relates to apparatus for measuring the concentration of a substance in a liquid and particularly, but not exclusively, to apparatus for measuring the concentration of glucose in blood or interstitial fluid.

Devices for measuring blood glucose levels are invaluable to diabetics, especially devices that can be used by patients themselves, as they can monitor their own glucose levels and take an appropriate dose of insulin. Therefore, the accuracy of such devices is very important, since improper reading could lead to the administration of an incorrect level of insulin, which could be very harmful.

It also happens that in all practical blood glucose measurement systems at least one part of the device, that is, the part that contacts the blood sample, is disposable. This means that it is particularly important that the cost can be minimized, particularly of any disposable part, since the user will usually need a large number thereof regularly.

Currently known devices for measuring glucose favor an electrochemical measurement procedure instead of the old colorimetric procedures. The general principle is the measurement of an electric current between sensor parts respectively called the working and reference sensor part. The working sensing part comprises an electrode on which an enzyme reagent layer comprising an enzyme and an electron-mediating compound has been deposited. When a potential is applied through the sensor parts, a current is generated, through the enzyme, due to the transfer of electrons between the substance being measured (the substrate above) and the electrode surface. The current generated is proportional to both the area of the sensor part and the concentration of glucose in the test sample. Since the area of the working sensing part is supposedly known, the electric current will be proportional to the glucose concentration.

It has been recognized in the art that if the working sensing part is not completely covered with blood, imprecise results are obtained, since the effective area is reduced. Various ways of addressing this problem have been proposed, two of which are disclosed in US 5628890 and US 5582697. These two procedures are based on a unidirectional flow of blood through the surface of the test strip, and both initiate the test measurement by detecting the presence of the sample liquid in an electrode or sensor part located downstream of the working sensor part.

International Patent WO9958709 discloses an improved, disposable test strip, which has three or more electrodes. This strip is designed so that different electrical potentials can be maintained, between a pseudo counter electrode or common reference electrode and each of the other electrodes, with the application of a common potential by means of an ammeter meter. This possibility is achieved by providing the test strip with different circuit resistors for each of these other electrodes.

European Patent EP0537761 provides a biosensor comprising an insulating substrate of electricity, a main electrode system formed on the substrate and having a working electrode and a counter electrode, a reaction layer in contact with the main electrode system or in the near it and containing an oxidoreductase, and a reference electrode subsystem located with an interval with respect to the main electrode system and having a working electrode and a counter electrode.

Finally, US Patent US5628890 describes an electrode strip that includes an electrode holder, a counter electrode or reference electrode disposed on the support, a working electrode located on the support and separated from the reference electrode or reference electrode, a coating layer which defines a closed space on the active and reference electrodes and that has an opening for a sample to penetrate the closed space, and a plurality of grid layers interposed in the closed space between the coating layer and the support, having the coating layer an opening for application of the separate sample of said electrodes, and said reference electrode being separated from said working electrode and located in a remote position from and on the opposite side of said working electrode of said opening. The working electrode includes an enzyme capable of catalyzing a reaction consisting of a substrate for the enzyme or a catalytically reactive substrate with an enzyme and a mediator capable of transferring the transferred electrons between the reaction catalyzed by the enzyme and the working electrode to create a current representative of the activity of the enzyme and representative of the compound.

Naturally, the problem that there is insufficient sample liquid, and therefore the working sensing part is not completely covered, can be reduced by decreasing the size of the working sensing part. However, a small area in the work sensing part tends to produce greater variability in the calibrated results.

The present inventors have verified that in addition to an incomplete coverage of the working sensing part, inaccurate results can also occur due to occasional defects in the manufacturing of the test strips for such devices, as well as by accidental damage caused to the sensing part. of work, for example, by a user. As far as the inventors know, the only practical way to deal with this problem has been to ensure that the printing process used to make the test strips is as accurate as possible and to have adequate quality control.

It is an object of the present invention to mitigate said inconveniences, at least partially.

The present invention provides a disposable test strip for measuring the concentration of a substance in a sample liquid, said test strip comprising a reference sensing part and a working sensing part for generating load carriers in proportion to the concentration of said substance in the sample liquid; wherein said test strip further comprises a second working sensing part for generating load carriers in proportion to the concentration of said substance in the sample liquid.

It will therefore be appreciated that, according to the fifth invention, the measuring device compares the current passing through the two working sensor parts, as a consequence of the generation of load carriers, and gives an error indication if

5 the two currents are too different, that is, if the current in one sensor part differs too much from what could be expected considering the current in the other. This procedure can not only detect when one of the sensor parts is not adequately covered by the sample liquid, but also detect if there is a manufacturing defect in either the sensor part or if it has been damaged after manufacture, since even With the working sensor parts completely covered, an abnormal current will arise in the affected sensor part in such circumstances.

According to the invention, the only type of defect or damage that would not necessarily be recognized is one that affects both of the working sensing parts in the same degree. However, this is logically less likely than an effect that affects a sensing part of individual work and is thus an improvement over the prior art. In practice such a probability is considered to be insignificant. In any case, the invention is not limited to providing only two

15 work sensing parts and the skilled person could therefore choose to provide three or more work sensing parts to further reduce the probability that they are all affected by an identical defect.

Seen in another way, the invention provides an arrangement whereby, for a given total area of the working sensing part, and therefore a given minimum volume of the sample, the detection of inadequate filling and defects can be obtained or damage to the working sensing part, dividing the area of the working sensing part in two.

The two working sensing parts are arranged downstream of each other. This makes it possible to ensure that one of the sensor parts will always be completely covered when the other begins to be covered, thus avoiding the possibility, even if it is remote, that there is not enough sample liquid to cover both sensor parts, and even that each sensor part is partially covered in the same magnitude. It will be appreciated, however, that although the aforementioned small risk is considered acceptable, the arrangements according to the invention allow flexibility in the

The placement of the sensor parts much greater than the known devices, while providing protection against the use of an inappropriate volume of sample liquid, or other incorrect use of the product, or damage to it. More preferably, both working sensing parts are located downstream of the reference sensing part.

Both sensor parts comprise the same work material. Additionally, both work sensing parts have the same area. The two working sensing parts are substantially identical. This allows the difference parameter to easily include a direct comparison between the respective currents that pass through the sensor parts to determine if a safe measurement of the substance concentration can be made.

The threshold used to determine an inaccurate measure can be chosen as appropriate. Typically the threshold will be empirically chosen, since the appropriate value will depend on the inherent variability of the manufacturing process, the desired accuracy for the results, etc. In a way there is a trade-off between the accuracy that can

35 obtained by setting a low threshold and the proportion of measurements that should be discarded because they are too inaccurate. Therefore, the threshold should be set advantageously at a level where, for example, no appreciable harm is done to a patient who depends on the results for administering insulin.

The difference parameter may be an absolute value, for example the difference between the measured currents in each sensor part, although preferably it is dimensionless, for example a percentage of one or the other of the measured currents.

Preferably the currents are measured after a predetermined time, although this is not essential.

The actual value of the current used to calculate the concentration of the substance may just be that of one of the working sensing parts, but a combination thereof, for example the sum or the average of the two, is preferable. This has the advantage that the effective work area is used to the maximum, which helps to increase the accuracy of

45 results obtained.

The invention, therefore, is directed to a device for measuring blood glucose concentration, in which the two working sensing parts and the reference sensing part are provided on a disposable test strip.

Brief description of the drawings:

Figure 1 shows a base member for a test strip according to the invention;

Figure 2 shows the arrangement of the carbon tracks applied on the base member;

Figure 3 shows the insulating layer applied on the strip;

Figure 4 shows the enzyme reagent layer;

Figure 5 shows a layer of adhesive;

Figure 6 shows a layer of hydrophilic film;

55 Figure 7 shows the coating layer of the strip;

Figure 8 is a graph of the results obtained without using a method according to the invention; and Figure 9 is a graph similar to Figure 8 obtained using a method according to the invention.

Returning to Figure 1, an oblong polyester strip 2 is shown that forms the basis of a test strip for measuring glucose concentration in a blood sample. The base element 2 is shown in isolation, although in practice, at the end of manufacturing, a set of such strips is cut from a large master sheet.

Figure 2 shows the carbon ink template that, in this example, is applied to the base element by screen printing, although any suitable deposition process known in the art could be used. The carbon layer comprises four distinct areas that are electrically isolated from each other. The first track 4 forms, at the distal end thereof, an electrode 4b for an opposite or reference sensor part. The track 4 extends longitudinally forming a connection terminal 4a at its proximal end. The second and third tracks 6, 8 form at their distal ends electrodes 6b, 8b for two working sensor parts and corresponding connection terminals 6a, 8a at their proximal ends. The fourth carbon area is simply a connecting bridge 10 that is provided to close the circuit in a suitable measuring device so that it is present when the test strip is correctly inserted.

Figure 3 shows the third layer, which is also applied by screen printing. It is a water-insoluble mask 12, which defines a window on the electrodes 4b, 6b, 8b and thus controls the size of the exposed carbon, and therefore the place where the enzyme reagent layer 14 (Figure 4 ) will contact carbon electrodes. The size and shape of the window are set so that the two electrodes 6b, 8b have an enzyme patch, of exactly the same area, printed on them. This means that, for a given potential, each working sensing part will theoretically let the same amount of electric current pass in the presence of a blood sample.

An enzyme layer is printed, which in this embodiment is a layer 14 of glucose oxidase reagent (Figure 4), on mask 12, and therefore on electrodes 4b, 6b, 8b through the window of the mask, to respectively form the opposite or reference sensing part and the two working sensing parts. Next, a 150 µm layer of adhesive is printed on the strip with the template shown in Figure 5. For clarity, this template is enlarged with respect to the previous figures. Three independent adhesive areas 16a, 16b, 16c define a sample chamber 18 together.

Two sections of hydrophilic film 20 (Figure 6) are laminated on the distal end of the strip (Figure 6) and fastened by eladhesive 16. The first film section has the effect of converting the sample chamber 18 into a thin capillary channel which draws the capillary action. liquid towards itself and along it. The final layer appears in Figure 7 and consists of a protective plastic tape 22 having a transparent part 24 at the distal end. This allows the user to instantly know if a strip is used and also helps to perform a rudimentary visual check of whether enough blood has been applied.

Next, the use of the strip will be described. The test strip is inserted into a measuring device. The storage part 10 completes a circuit of the device and automatically turns on the device. The device also has some contacts to connect to terminals 4a, 6a, 8a of the strip. The measuring device applies a potential of 400 mV between the opposite or reference sensor part and each of the two working sensor parts through the aforementioned terminals.

A drop of blood is then placed on the distal end of the strip. The capillary action carries the blood along the sample chamber 18 and over the opposite or reference sensor part and the two working sensor parts.

After a predetermined time, the current passing through each working sensing part is measured and the two measurements are compared. If they differ by more than 10%, an error message is displayed on the measuring device, and the test must be repeated. However, if they are within 10% of each other, the two currents in the device are added and converted into a glucose level that is displayed in a liquid crystal display.

A comparative experiment was performed using a strip, manufactured as indicated, in order to exemplify the achievable benefits according to the invention. During the experiment, a few drops of blood were applied on such strips, the volume of which increased from 1 to 2 microliters, to 0.2 microliter steps, and with a constant swallowing concentration, repeating each volume 8 times. The current in each work sensor part was measured and recorded. The results are reflected in Table 1 attached to this description.

In the first part of the test, the two currents were simply added to simulate a single sensor working part that had the combined areas. These results are drawn in Figure 8.

In the second half of the test, the two currents were first compared. Only if they differed by less than 10% were they added to each other and given as a valid result. Values that differed by more than 10% were discarded. The results of this second part are drawn in Figure 9.

It is immediately apparent that the second set of results is significantly more precise, that is, it has a much smaller variation. In addition, since in practice the two working sensing parts will only give mutually consistent results if both are fully covered, the second set of results is also significantly more accurate than the first, since it can be assumed without risk that the results are only given. really when both working sensing parts are fully covered.

It will therefore be appreciated that the present invention allows the detection and rejection of those tests that have had insufficient blood applied to the test strip, that is, those in which the test strip was used incorrectly. Likewise, the detection and rejection of test strips that are defective, whether due to damage or manufacturing defect, will also be allowed.

Those skilled in the art will appreciate that many variations on the above described are possible within the scope of the invention. For example, the invention can be used to measure the level of any substance in any liquid, not just blood glucose.

In addition, the 10% difference figure used in the above described embodiment is purely exemplary, and any suitable figure may be used.

Table 1

ΜL volume

Work sensor 1: µA Work sensor 2: µA % difference With error detection No error detection

one

7.07 0.00 -706800 7.07

one

6.94 5.98 -16,2175732 12.92

one

5.53 0.01 -92050 5.54

one

6.99 7.09 1.42393909 14.09 14.09

one

7.34 7.02 -4,59016393 14.35 14.35

one

7.16 6.79 -5.49742078 13.94 13.94

one

7.01 3.47 -102,13441 10.48

one

7.07 5.69 -24,2578605 12.77

1.2

7.18 4.54 -58,2286847 11.72

1.2

7.00 6.78 -3,35055351 13.78 13.78

1.2

7.09 1.79 -297,032475 8.88

1.2

6.31 0.00 -157550 6.31

1.2

6.78 6.79 0,11788977 13.56 13.56

1.2

6.95 6.59 -5,4029443 13.53 13.53

1.2

6.62 6.28 -5,36795158 12.89 12.89

1.2

7.23 3.78 -91.2721502 11.01

1.4

7.16 6.90 -3,76811594 14.06 14.06

1.4

7.14 6.94 -2.88184438 14.08 14.08

1.4

7.17 7.02 -2,13675214 14.19 14.19

1.4

7.02 6.01 -1.5918958 13.93 13.93

1.4

6.95 6.91 -0.5788712 13.86 13.86

1.4

6.93 6.88 -0.72674419 13.81 13.81

1.4

7.09 6.92 -2.4566474 14.01 14.01

1.4

7.25 7.40 2,02702703 14.65 14.65

1.6

7,808 6.59 -18.4825493 14.40

1.6

6,774 6,589 -2,80770982 13.36 13.36

1.6

6,928 6,904 -0.34762457 13.83 13.83

1.6

6,892 6,453 -6.80303735 13.35 13.35

1.6

7,087 7,314 3,10363686 14.40 14.40

(continuation)

1.6

7,257 6,947 -4,46235785 14.20 14.20

1.6

6,501 6,306 -3.09229305 12.81 12.81

1.6

6,811 6,755 -0.82901554 13.57 13.57

1.8

7,145 6,536 -9,31762546 13.68 13.68

1.8

7,021 6,612 -6.18572293 13.63 13.63

1.8

6,917 6,828 -1,30345636 13.75 13.75

1.8

6,971 6.78 -2,81710914 13.75 13.75

1.8

7,016 6,941 -1,08053595 13.96 13.96

1.8

6,977 7,179 2,81376236 14.16 14.16

1.8

6,946 6,794 -2,23726828 13.74 13.74

1.8

7,203 7,183 -0.27843519 14.39 14.39

2

7,145 6,536 -9,31762546 13.68 13.68

2

7,021 6,621 -6.18572293 13.63 13.63

2

6,917 6,828 -1,30345636 13.75 13.75

2

6,971 6.78 -2,81710914 13.75 13.75

2

7,016 6,941 -1,08053595 13.96 13.96

2

6,977 7,179 2,81376236 14.16 14.16

2

6,946 6,794 -2,23726818 13.74 13.74

2

7,203 7,183 -0.27843519 14.39 14.39

Claims (5)

1. A disposable test strip for measuring the concentration of a substance in a sample liquid, said device comprising: A first active sensor working area to generate load carriers in proportion to the concentration of said substance in the sample liquid; said first active working sensor area comprises an electrode (8b); a second active working sensor area, said second active working sensor area comprises an electrode (6b); and a reference sensor (4b) located upstream of said first and second active working sensing areas, whose reference sensor is a common reference for both the first and second active working sensing areas, characterized in that said second active area working sensor is located downstream of said first active working sensor area and generates load carriers in proportion to the concentration of said substance in the sample liquid, said first and second active sensor working areas are substantially identical, such that in the absence of an error condition, the amount of said load carriers generated by said first active sensor working area is substantially identical to the amount of said load carriers generated by said second active sensor work area and, said reference sensor and said first and second working sensor parts, arranged such that the sample liquid is forced to flow substantially in one direction through said reference sensor and said first and second active working sensor areas.

2.

 The disposable test strip claimed in claim 1, arranged so that the sample liquid is forced to flow substantially in one direction.

3.

The disposable test strip claimed in claim 1 arranged to measure said currents after a predetermined time after application of the sample.

Four.

 The disposable test strip of claim 1 further comprises:

A base member, in which the two active sensor work areas are provided on the member should.

5.

The disposable test strip as claimed in claim 1 or 4, wherein the substance to be measured is glucose, and each of the active sensing work areas generates charge carriers in proportion to the concentration of glucose in the sample liquid.