Determination of Partial Fill in Electrochemical Strips

• US 20060231418A1
• Filed: 04/15/2005
• Published: 10/19/2006
• Est. Priority Date: 04/15/2005
• Status: Active Grant

• Current Assignee: Agamatrix Incorporated (AgaMatrix Holdings LLC d/b/a WaveForm Diabetes)
• Original Assignee: Agamatrix Incorporated (AgaMatrix Holdings LLC d/b/a WaveForm Diabetes)
• Inventors: Iyengar, Sridhar G., Diamond, Steven, Harding, Ian, Williams, Richard
• Granted Patent: US 7,547,382 B2
• Time in Patent Office: Days
• US Class Current: 205/775
• CPC Class Codes: G01N 27/327 Biochemical electrodes , e.g. electrical or mechanical details for in vitro measurements

Abstract

Partial fill of an electrochemical test strip is determined by making a DC determination of double layer capacitance from charging or discharging charge on a test strip containing sample, for example a blood sample to be tested for glucose. The measured double layer capacitance is compared to a reference value. The double layer capacitance may be determined as an integral or differential capacitance. Double layer capacitance may also be used for quality control to monitor the quality of electrode formation, particularly in strips using screen printed electrodes.

Claims

1. A method for detection of partial fill in an electrochemical test strip having electrodes and a liquid sample disposed between the electrodes comprising the steps of:

(a) introducing sample to an electrochemical test strip;

(b) applying a potential difference, V_app, between the electrodes of the test strip;

(c) switching off the applied potential at time t_switch and optionally reapplying a second potential;

(d) observing current generated and determining from the observed current a double layer charging or discharging at the electrodes;

(e) observing a voltage change after the applied potential is switched off, and determining the double layer capacitance of test strip from the measured double layer charging or discharging and the observed voltage change; and

(f) comparing the determined double layer capacitance to a reference value, wherein a double layer capacitance less than the reference value is an indication that the liquid sample covers a portion of the electrodes and that the electrochemical test strip is only partially filled.

2. The method of claim 1, wherein double layer capacitance determined is an integral capacitance determined in accordance with the formula:

3. The method of claim 2, wherein Dt is determined by monitoring the decay in the potential difference between the electrodes to identify the time, t_threshold required for the potential to decay to a threshold value, V_threshold, and wherein Dt=t_threshold−

t_switch.

4. The method of claim 3, wherein the amount of double layer charge discharged is determined as

charge=I_switchΔ

t

where I_switch is the current just before time t_switch.

5. The method of claim 3, wherein the current is an estimated value of current at time t_meas determined by extrapolation from the observed current prior to t_switch.

6. The method of claim 3, wherein Δ

V is the difference between V_app and V_threshold.

7. The method of claim 3, wherein an immediate drop in voltage, V_drop, is observed following switching off of potential, and Δ

V is given by (V_app−

V_drop)−

V_threshold.

8. The method of claim 2, wherein the observed current occurs after reapplication of a potential, and the double layer capacitance is determined from double layer charging.

9. The method of claim 1, wherein a differential capacitance is determined according to the formula:

C_dif=I/(dV/dt),

wherein I is current and (dV/dt) is the instantaneous change in voltage at time t_meas.

10. The method of claim 9, wherein the current I is the current just before time t_switch.

11. The method of claim 9, wherein the current is an estimated value of current at time t_meas determined by extrapolation from the observed current prior to t_switch.

12. The method of claim 7, wherein t_meas is determined dynamically.

13. The method of claim 12, wherein t_meas is the time at which the observed potential is lower than the applied potential by a predetermined amount.

14. The method of claim 8, wherein an immediate drop in voltage, V_drop, is observed following switching off of potential, and wherein t_meas is the time at which the observed potential is lower than (V_app−

V_drop) by a predetermined amount.

15. The method of claim 14, wherein the current I is the current just before time t_switch.

16. The method of claim 14, wherein the current is an estimated value of current at time t_meas determined by extrapolation from the observed current prior to t_switch.

17. The method of claim 1, wherein the measured double layer capacitance is corrected by an additive correction term that is a function of temperature and oxygen carrying capacity prior to comparison with the reference value.

18. A meter for receiving an electrochemical test strip having electrodes and providing a determination of an analyte in a sample applied to the electrochemical test strip when received in the meter, said meter comprising

(a) a housing having a slot for receiving an electrochemical test strip;

(b) communications means for receiving input from and communicating a result to a user; and

(c) means for making a DC determination of double layer capacitance on a test strip containing sample received within the meter, and comparing the determined double layer capacitance to a reference value, wherein a double layer capacitance less than the reference value is an indication that the liquid sample covers a portion of the electrodes and that the electrochemical test strip is only partially filled.

19. The meter of claim 18, wherein said means for making a DC determination of double layer capacitance comprises circuitry and a processor programmed to:

(i) apply a potential difference, V_app, between the electrodes of the test strip;

(ii) switch off the applied potential at time t_swtich and optionally reapply a second potential at a subsequent time;

(iii) observe current generated and determine from the observed current a double layer charging or discharging at the electrodes; and

(iv) observe a voltage change after the applied potential is switched off, and determine the double layer capacitance of test strip from the measured double layer charging or discharging and the observed voltage change.

20. The meter of claim 19, wherein double layer capacitance determined is an integral capacitance determined in accordance with the formula:

21. The meter of claim 20, wherein Δ

t is determined by monitoring the decay in the potential difference between the electrodes to identify the time, t_threshold required for the potential to decay to a threshold value, V_threshold, and wherein Δ

t=t_threshold−

t_switch.

22. The meter of claim 21, wherein the amount of double layer charge discharged is determined as

charge=I_switchΔ

t

where I_switch is the current just before time t_switch.

23. The meter of claim 21, wherein the current is an estimated value of current at time t_meas determined by extrapolation from the observed current prior to t_switch.

24. The meter of claim 21, wherein Δ

V is the difference between V_app and V_threshold.

25. The meter of claim 21, wherein an immediate drop in voltage, V_drop, is observed following switching off of potential, and Δ

V is given by (V_app−

V_drop)−

V_threshold.

26. The meter of claim 19, wherein the observed current occurs after reapplication of a potential, and the double layer capacitance is determined from double layer charging.

27. The meter of claim 19, wherein a differential capacitance is determined according to the formula:

C_dif=I/(dV/dt),

wherein I is current and (dV/dt) is the instantaneous change in voltage at time t_meas.

28. The meter of claim 27, wherein the current I is the current just before time t_switch.

29. The meter of claim 27, wherein the current is an estimated value of current at time t_meas determined by extrapolation from the observed current prior to t_switch.

30. The meter of claim 27, wherein t_meas is determined dynamically.

31. The meter of claim 30, wherein t_meas is the time at which the observed potential is lower than the applied potential by a predetermined amount.

32. The meter of claim 27, wherein an immediate drop in voltage, V_drop, is observed following switching off of potential, and wherein t_meas is the time at which the observed potential is lower than (V_app−

V_drop) by a predetermined amount.

33. The meter of claim 32, wherein the current I is the current just before time t_switch.

34. The meter of claim 32, wherein the current is an estimated value of current at time t_meas determined by extrapolation from the observed current prior to t_switch.

35. A measurement system comprising a meter in accordance with claim 19, and an electrochemical test strip disposed within the housing.

36. The measurement system of claim 35, wherein the electrochemical test strip measures glucose in a sample.

37. A method for quality inspection of a lot of electrochemical test strips, comprising the steps of:

(a) obtaining a plurality of test strips from the lot;

(b) applying a sample to each of the test strips;

(c) measuring the double-layer capacitance of the test strips in the presence of the sample; and

(d) determining the variability in measured double-layer capacitance, wherein a variability in excess of a defined threshold indicates a quality deficiency in the test strips.