METHOD FOR DETERMINING DIFFUSION

Abstract

A method for determining a diffusion feature of a fluidic sample using redox reactions in an electrochemical cell that has at least two electrodes, wherein the first electrode has at least one redox mediator at its surface or in close vicinity of its surface, and the second electrode has an electrode surface free of the redox mediator(s) in the beginning of a test, the method comprising: applying an electric potential to a fluidic sample in the electrochemical cell to initiate redox reactions at the two electrode surfaces; measuring current associated with the applied potential as a function of time, and using a measurement point on a transient part of the measured current at or after a turning point and its associated time to determine the diffusion feature.

Claims

1. A method for determining a diffusion feature of a fluidic sample using redox reactions in an electrochemical cell that has at least two electrodes, wherein the first electrode has at least one redox mediator at its surface or in close vicinity of its surface, and the second electrode has an electrode surface free of the redox mediator(s) in the beginning of a test, the method comprising: applying an electric potential to a fluidic sample in the electrochemical cell to initiate redox reactions at the two electrode surfaces; measuring current associated with the applied potential as a function of time, and using a measurement point on a transient part of the measured current at or after a turning point and its associated time to determine the diffusion feature.

2. A method as claimed in claim 1, wherein the turning point is the point at which a first transient part ends and a second transient part begins, wherein the first transient part deviates from a Cottrell current decay and the second transient part substantially follows a Cottrell current decay.

3. A method as claimed in claim 2 comprising identifying the turning point on the transient part of the measured current.

4. A method as claimed in claim 3 comprising using the identified turning point to determining the measurement point that is to be used for determining the diffusion feature, wherein the measurement point is offset in time from the turning point (t.sub.turn+Δt, =identified point).

5. A method as claimed in claim 4 comprising using the measurement point current and its associated test time to determine simultaneously the diffusion feature and contribution of at least one redox-active substance.

6. A method as claimed in claim 5 further comprising storing calibration information and using the calibration information together with the measurement point in the measured current and its associated test time to determine the diffusion feature.

7. A method as claimed in claim 6 further comprising storing a function that relates a test time and the diffusion feature and using the function together with the measurement point current and its associated test time to determine the diffusion feature.

8. A method as claimed in claim 7, wherein the diffusion feature is a diffusion property or characteristic associated with mass transfer in a fluidic sample, including diffusion coefficient, haematocrit, viscosity.

9. A method as claimed in claim 8, wherein the redox-active substance is at least one substance which undergoes redox reaction at the second electrode under the applied electric potential.

10. A method as claimed in claim 9, wherein the electric potential has a constant magnitude, or a varying magnitude with time, or combination of a constant magnitude and a varying magnitude.

11. A method as claimed in claim 10, wherein the current is a single value of the measured current, or an average value of consecutive measured currents over a duration of 0.5 second, preferably 0.1 second, more preferably 0.03 second.

12. A method as claimed in claim 11, wherein the electrodes are configured in a co-facial manner such that the two electrode surfaces are spatially arranged facing each other with a minimum face-to-face distance of 10 to 1000 microns, preferably 35 to 500 microns, and more preferably 50 to 120 microns.

13. A method as claimed in claim 11, wherein the electrodes are configured in a co-planar manner such that the two electrode surfaces are spatially arranged in the same plane with the minimum edge-to-edge distance of 10 to 2000 microns, preferably 50 to 900 microns, and more preferably 100 to 500 microns.

14. A meter for determining a diffusion feature of a fluidic sample using redox reactions in an electrochemical cell that has at least two electrodes, wherein the first electrode has at least one redox mediator at its surface or in close vicinity of its surface, and the second electrode has an electrode surface free of the redox mediator(s) in the beginning of a test, the meter being configured to: apply an electric potential to a fluidic sample in the electrochemical cell to initiate redox reactions at the two electrode surfaces; measure current associated with the applied potential as a function of time; and use a measurement point on a transient part of the measured current at or after a turning point and its associated time to determine the diffusion feature.

15. A meter as claimed in claim 14, wherein the turning point is the point at which a first transient part ends and a second transient part begins, wherein the first transient part deviates from a Cottrell current decay and the second transient part substantially follows a Cottrell current decay.

16. A meter as claimed in claim 15 configured to identify the turning point on the transient part of the measured current.

17. A meter as claimed in claim 16 configured to use the identified turning point to determine the measurement point that is to be used for determining the diffusion feature, wherein the measurement point is offset in time from the turning point (t.sub.turn+Δt, =measurement point).

18. A meter as claimed in claim 17 configured to use the measurement point current and its associated test time to determine simultaneously the diffusion feature and contribution of at least one redox-active substance.

19. A meter as claimed in claim 18 configured to store calibration information and use the calibration information together with the measurement point in the measured current and its associated test time to determine the diffusion feature.

20. A meter as claimed in claim 19 configured to store a function that relates a test time and the diffusion feature and use the function together with the measurement point current test time to determine the diffusion feature.

21. A meter as claimed in claim 20, wherein the diffusion feature is a diffusion property or characteristic associated with mass transfer in a fluidic sample, including diffusion coefficient, haematocrit, viscosity.

22. A meter as claimed in claim 21, wherein the redox-active substance is at least one substance which undergoes redox reaction at the second electrode under the applied electric potential.

23. A meter as claimed in claim 22 configured to apply an electric potential that has a constant magnitude, or a varying magnitude with time, or a combination of a constant magnitude and a varying magnitude.

24. A meter as claimed in claim 23, wherein the current is a single value of the measured current, or an average value of consecutive measured currents over a duration of 0.5 second, preferably 0.1 second, more preferably 0.03 second.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Various aspects of the invention will now be described by way of example only, and with reference to the following drawings, of which:

[0028] FIG. 1 shows a redox reaction at two electrodes stimulated by applying an electrical potential between the electrodes;

[0029] FIG. 2 shows a plot of current versus time for the device of FIG. 1 measured after application of the electrical potential, on which a turning point at time

[0030] FIG. 3a is a box plot of t.sub.turn versus glucose, uric acid and Hct for blood samples of two donors;

[0031] FIG. 3b is a box plot of t.sub.turn versus Hct data of FIG. 3a, and

[0032] FIG. 4 is a box plot of i.sub.turn versus Hct and uric acid concentration at a glucose concentration around 75 mg/dL.

DETAILED DESCRIPTION OF THE DRAWINGS

[0033] FIG. 1 shows redox reactions of an electrochemical test strip, for example a self-monitoring blood glucose strip. The strip has two electrodes, a first electrode E1 and second electrode E2. The first electrode E1 is covered with a reagent layer which contains redox mediator (M) and other materials (e.g. enzyme) while the second electrode E2 has a surface without the covering reagent layer. The first and second electrodes E1 and E2 respectively are electrically connected to a potentiostat (not shown). In use, the first and second electrodes E1 and E2 respectively are in contact with a whole blood sample and an electric potential (voltage) is applied between the two electrodes. This results in redox reactions at the both electrodes. The resulting current between the first and second electrodes E1 and E2 is measured as a function of time.

[0034] To test the strip, an electric potential is applied between the first and second electrodes E1 and E2, and the current flow is measured. The magnitude and polarity of the electric potential are chosen to initiate a reduction(s) of the mediator(s) at the first electrode E1 and an oxidation(s) of redox-active substance(s) at the second electrode E2. Applying a blood sample to the strip sample chamber triggers physical and chemical processes/changes which depend on Hct and redox-active substance(s) of the blood sample. The physical processes include hydration of the reagent layer, dissolution of the mediator, and double-layer charging (a process to neutralize the charge imbalance near the electrode surfaces by rearrangement of charged species in the blood). The chemical processes include oxidation(s) of the redox-active substance(s) at the second electrode E2 and reduction of the oxidised mediator M.sub.ox at the first electrode E1, as shown in FIG. 1.

[0035] As a result of the physical and chemical processes, the recorded current has a transient that has a unique pattern which deviates from Cottrell current decay. This is shown in FIG. 2. Each current transient has a “turning-point”, as marked “X” in FIG. 2, which defines a current parameter i.sub.turn and a time parameter t.sub.turn. The turning point is the point at which a first transient part with low level oscillations ends, and a second transient part with a smooth current decay begins (NB in FIG. 2 the current shown is a negative current and so the rising slope to the right of the low level oscillations represents a current decay). The first transient part deviates from Cottrell current decay while the second transient part substantially follows Cottrell current decay. The second transient part ends as soon as the current reaches a steady state or the redox mediator arrives at the second electrode E2 by diffusion from the reagent layer. The turning point may be identified by a process(es)/algorithm(s) which may be developed using various mathematical approaches/techniques.

[0036] The deviation of the current transient from Cottrell current decay, in particular the first transient part, mainly results from the physical processes that play a predominant role at this stage in changing the active surface area of the first electrode E1 and/or availability of the mediator for the reduction at the first electrode E1. These physical processes are dependent on diffusion of the blood sample and independent of the redox-active substance(s) concentration. Therefore, the time at which the transient current transitions from the first transient part to the second transient part, t.sub.turn, is a function of diffusion.

[0037] At an early stage of a strip test, the reduced mediator has not diffused across the sample chamber to reach the surface of the second electrode E2. Hence, the oxidation current is predominantly generated by the oxidation of redox-active substance(s). At the same time, the oxidation of redox-active substance(s) is dependent on mass transfer of the redox-active substance(s) in the fluidic sample. Therefore, i.sub.turn is a function of both the redox-active substance(s) and its diffusion.

[0038] The function for t.sub.turn and the function for i.sub.turn may be derived from laboratory data obtained by testing fluidic samples with designated diffusion property and redox-active substance(s). In the present invention, testing a fluidic sample with an electrochemical cell, recording the current transient, identifying the turning point, defining t.sub.turn and i.sub.turn, and solving the two functions/equations enables simultaneous determinations of diffusion and the contribution/impact of redox-active substances of the fluidic sample.

[0039] The determination/measurement refers to quantitative, semi-quantitative, or qualitative evaluation of a diffusion property of the test sample (e.g. Hct) or a redox-active substance(s) (e.g. uric acid) in the test sample. For quantitative and semi-quantitative evaluation, the result is a numerical representation of the signal generated by one or more of the diffusion property or the redox-active substances present in the test sample. As a measure of the redox-active substance(s), it quantifies overall contribution of the redox-active substances.

[0040] The method is also applicable to simultaneously measuring diffusion and any redox-active substance(s) which undergoes reduction at the second electrode E2. In this case, the reagent layer at the first electrode E1 contains mediator in its reduced state and an electric potential is applied to initiate a reduction(s) of the redox-active substance(s) at the second electrode E2.

[0041] To allow diffusion dependent features or features that have an impact on diffusion to be measured in accordance with the invention, the diffusion feature of interest has to be calibrated as a function of the turning time t.sub.turn. Alternatively, the diffusion feature of interest could be represented by a mathematical function that is dependent on the turning time t.sub.turn. In either case, once the relationship between the diffusion feature and the turning time t.sub.turn is known, it can be used in later measurements to provide a measure of the diffusion feature. The diffusion feature may be, for example, diffusion coefficient, haematocrit (which impacts diffusion), coagulation or viscosity.

[0042] To allow the contribution of any redox-active substance(s) to be measured, the redox-active substance(s) of interest has to be calibrated as a function of the turning time t.sub.turn and additionally as a function of turning current i.sub.turn. Alternatively, the redox-active substance(s) of interest could be represented by a mathematical function that is dependent on the turning time t.sub.turn and the turning current i.sub.turn. In either case, once the relationship between the redox-active substance(s), the turning time t.sub.turn and the turning current i.sub.turn is known, it can be used in later measurements to provide a measure of the redox-active substance(s) or of the contribution to the measured current made by the redox-active substance(s).

[0043] The measure of the redox-active substance(s) may be a measure of the substance concentration in the sample. The measure of the contribution made by the redox-active substance(s) may be a measure of the contribution to the current. This can be used in subsequent steps or processes to correct any calculations based on the current measurements, when such calculations require the effects of the redox-active substance(s) to be excluded. For example, uric acid interferes with electrochemical glucose measurements, and the invention would allow the effects of the uric acid to be identified and excluded from any calculation of glucose levels.

[0044] The methodology of the invention has been tested using commercially available strips. Tests were done at room temperature. A potential of −400 mV was applied and the resultant current signals recorded. Two blood samples were used from two different donors, each with a different glucose level. The samples were 2×6×4 (glucose/donor×Hct×uric acid) manipulated venous blood. The tests were repeated eight to ten times for each blood sample. The results of these tests are represented graphically in FIGS. 3a, 3b and 4.

[0045] FIG. 3a is a box plot of t.sub.turn versus various blood sample variables, including glucose concentration, spiked/added uric acid concentration (a redox-active substance that interferes with the glucose measurement) and Hct (which impacts diffusion). In this example, samples from two different donors were used, so that samples with two different glucose concentrations could be tested (on the x-axis of FIG. 3a “1” and “2” are for Donor1 and Donor2 respectively). In this case, the glucose concentrations were approximately 75 mg/dL for Donorl and 125 mg/dL for Donor2. The native (before spiking) uric acid concentrations were 4.47 mg/dL for Donorl and 4.79 mg/dL for Donor2 respectively. FIG. 3b is a box plot of t.sub.turn versus Hct. This shows the same Hct data as in FIG. 3a, but without the glucose and uric acid data for clarity.

[0046] FIGS. 3a and 3b clearly indicate a strong correlation between t.sub.turn and Hct with a good resolution, whilst glucose and uric acid have little effect on t.sub.turn. A mathematical calibration equation can be derived from the data (e.g. those in FIG. 3b) and used to provide a measurement of diffusion d or a diffusion related property (e.g. Hct) upon testing a fluidic sample. The calibration equation can be expressed as:

t.sub.turn=f(d)   [1]

[0047] FIG. 4 is a box plot of i.sub.turn versus Hct and uric acid concentration at a glucose concentration around 75 mg/dL (Donorl data). The plot clearly indicates a strong correlation between i.sub.turn and Hct with a good resolution. The plot also indicates a clear correlation between i.sub.turn and uric acid concentration at different Hct levels. A mathematical calibration equation can be derived from laboratory data (e.g. those in FIG. 4). The calibration equation can be expressed as:

i.sub.turn=f(d, r)   [2]

[0048] Measurements of diffusion d or a diffusion related property (e.g. Hct) and the contribution of redox-active substance(s) r (e.g. uric acid) can be obtained by solving equations [1] and [2]. Where multiple redox-active substances are present, the contribution determined may be a measure of the current that is attributable to more than one redox-active substance. This measure may be used as a correction factor for any measurements/calculations that are dependent on current. Where only a single redox-active substance is present, equations [1] and [2] can be used to determine the concentration of the redox-active substance in the fluid sample.

[0049] The invention has been described with reference to identifying the current and time parameters at the “turning-point”. However, a current parameter and/or a time parameter can be obtained from the other parts of the current transient, in particular after the “turning-point”. For example, a current parameter and a time parameter could be used from the section of the current response marked Y in FIG. 2. Indeed any current and associated time parameter from the second transient part could be used.

[0050] An advantage of using the turning point is that the current and time can be easily identified using simple processing techniques. If current and time parameters away from the turning point are used, their location could be identified with reference to the turning point. For example, if it were decided that the optimum current and time parameters were at a time t.sub.turn+Δt, then the parameters could be identified by firstly identifying t.sub.turn and using this as a reference to identify the time t.sub.turn+Δt.

[0051] The present invention provides a simple and effective technique for measuring bothdiffusion (e.g. Hct) and the contribution/impact of redox-active substance(s) (e.g. interference) in fluidic samples (e.g. blood) at the same time. It can be used for a variety of applications (e.g. Hct, viscosity, coagulation, etc.). It can also be used to determine correction factors to improve the accuracy of self-monitoring blood glucose tests by mitigating DIF (e.g. Hct) and RIF (e.g. uric acid), which are the two major error sources for episodic blood glucose monitoring. In this case, the methodology of the invention is independent of the glucose concentration in the blood samples. Hence, it is more reliable and robust than many prior art techniques. A further advantage is that there are no fundamental limitations on the strip sample chamber height, thereby reducing measurement errors resulting from the strip-to-strip and/or batch-to-batch variations.

[0052] A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. For example, although the invention has been described with reference to a test strip with only two electrodes, the invention could equally be applied to strips with three or more electrodes. Also, whilst the test strip of FIG. 1 has parallel plate electrodes (i.e. co-facial electrode configuration), the invention is applicable to strips with co-planar electrode configuration, i.e. all the electrode surfaces are in the same plane. In addition, although the signal of FIG. 2 is a response to a potential that has a constant magnitude, the applied potential could have a magnitude that varies with time, or could have a combination of a constant magnitude and a varying magnitude. Accordingly the above description of the specific embodiment is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.