Patent Application
20100327886
The present invention relates to a measurement device, a measurement system, and a concentration measurement method, for measuring the concentration of a target substance in a liquid sample.
Measurement systems that make use of a redox reaction (oxidoreduction) involving an enzyme have been proposed as conventional devices for measuring blood and other such liquid samples.
These measurement systems include a biosensor that has an insulated substrate, a working electrode and a counter electrode formed on the insulated substrate, a reaction layer provided on the working electrode and the counter electrode, and the reaction layer contains an oxidoreductase and an electron mediator, a substrate of the oxidoreductase is a target substance; and a system that applies an electrical voltage to between the working electrode and the counter electrode, detects an electrical current generated by the electrical voltage, and measures a glucose concentration based on the electrical current.
However, with a measurement system such as this, the current value may fluctuate depending on the presence of interfering substances such as compounds that are readily oxidized (such as ascorbic acid and uric acid), so accurate concentration measurement is sometimes difficult.
In view of this, in order to obtain an accurate concentration of the target substance, it has been proposed to correct a measurement result of the target substance by using a value of current obtained as follows: using a biosensor that is equipped with a first electrode system for detecting the value of current flowing with a redox reaction of a blood component, a second electrode system for detecting the blood cell count, and a third electrode system for detecting an amount of interfering substances, and in which at least a mediator is not disposed over the working electrode of this third electrode system, and applying voltage of one voltage value to the third electrode system, and obtaining a current value that reflects the concentration of interfering substances (Patent Citations 1 and 2).
Patent Citation 1: International Laid-Open Patent Application 2005-103669
Patent Citation 2: Japanese Laid-Open Patent Application H11-344462
However, the effect of interfering substances sometimes could not be eliminated with a conventional system due to factors such as the fact that the readily oxidizable potential varies from one kind of interfering substance to the next.
Accordingly, there has been a need for a device and method with which a target substance in a liquid sample could be more accurately measured.
To achieve this object, the first measurement device can be used for measuring the concentration of a target substance in a sample deposited on a biosensor having first and second electrode systems, the measurement device having: a mounting component configured to be mounted with the biosensor; a voltage application component configured to perform a first voltage application operation of applying a first voltage to the first electrode system and a second voltage application operation of applying at least two kinds of second voltage of mutually different levels at mutually different timings to the second electrode system; a concentration acquisition component configured to acquire the concentration of the target substance on the basis of the current value of the first electrode system when the first voltage is applied; and a correction component configured to correct the concentration acquired by the concentration acquisition component on the basis of the amount of change in the current value of the second electrode system between the at least two different timings during the second voltage application operation.
The second measurement device can be used for measuring the concentration of a target substance in a sample deposited on a biosensor having first and second electrode systems, the measurement device having: a mounting component configured to be mounted with the biosensor; a voltage application component configured to perform a first voltage application operation of applying a first voltage to the first electrode system, and a second voltage application operation of applying a second voltage that is a sweep voltage; a concentration acquisition component configured to acquire the concentration of the target substance on the basis of the current value of the first electrode system when the first voltage is applied; and a correction component configured to correct the concentration acquired by the concentration acquisition component on the basis of the amount of change in the current value of the second electrode system attributable to sweep of the second voltage.
Also, the first measurement system comprises a biosensor and a measurement device.
Also, the first concentration measurement method can be used in a measurement device for measuring the concentration of a target substance in a sample deposited on a biosensor having first and second electrode systems, the method having: a first voltage application step of applying a first voltage to the first electrode system, a concentration acquisition step of acquiring the concentration of the target substance on the basis of the current value of the first electrode system in the first voltage application step, a second voltage application step of applying at least two kinds of second voltage of mutually different levels at mutually different timings to the second electrode system, and a correction step of correcting the concentration acquired in the concentration acquisition step on the basis of the amount of change in the current value of the second electrode system between the at least two different timings during the second voltage application step.
Also, the second concentration measurement method can be used in a measurement device for measuring the concentration of a target substance in a sample deposited on a biosensor having first and second electrode systems, the method comprising a first voltage application step of applying a first voltage to the first electrode system, a concentration acquisition step of acquiring the concentration of the target substance on the basis of the current value of the first electrode system in the first voltage application step, a second voltage application step of applying a second voltage that is a sweep voltage to the second electrode system, and a correction step of correcting the concentration acquired in the concentration acquisition step on the basis of the amount of change in the current value of the second electrode system attributable to sweep of the second voltage.
An overview of the measurement system 100 pertaining to this embodiment will be described through reference to
As shown in
The liquid sample referred to here is not limited to any special kind of sample, and various samples can be used, such as blood, sweat, urine, and other such biological liquid samples; liquid samples originating in the environment, such as from a river, the sea, or a lake; and liquid samples originating in foods.
Nor is the target substance to be measured limited to any special kind of substance, and various substances can be accommodated by changing the oxidoreductase, etc., in the reagent layer 12 (discussed below) of the biosensor 10.
The structure of the biosensor 10 will be described in detail through reference to the exploded oblique view in
As shown in
The substrate 11 has an insulated substrate 111 composed of polyethylene terephthalate or the like, and an electrode layer formed on the surface thereof. The electrode layer is divided up with a laser or the like to form a first working electrode 11a, a second working electrode 11b, a detecting electrode 11c, and a counter electrode 11d. More specifically, the first working electrode 11a, the second working electrode 11b, the detecting electrode 11c and the counter electrode 11d are formed so that the end of the counter electrode 11d, the end of the second working electrode 11b, the end of the first working electrode 11a, and the end of the detecting electrode 11c are disposed in that order along the lengthwise direction of a sample supply path 10b (discussed below), from near the sample introduction port 10a and heading away from the sample introduction port 10a. Part of the counter electrode 11d is also disposed between the second working electrode 11b and the first working electrode 11a in the lengthwise direction of the sample supply path 10b.
As discussed below, the first working electrode 11a and the counter electrode 11d function as a first electrode system used in the detection of a target substance, and the second working electrode 11b and the counter electrode lid function as a second electrode system used in the detection of interfering substances.
The reagent layer 12 is a layer that contains an enzyme, and is disposed so as to be in contact with the first working electrode 11a, the detecting electrode 11c and the counter electrode 11d. The reagent layer 12 is disposed so as not to be in contact with the second working electrode 11b. The reagent layer 12 may contain an electron mediator or other such component in addition to the enzyme.
An oxidoreductase in which the target substance is the substrate can be used favorably as the enzyme. Examples of such oxidoreductases include glucose oxidase or glucose dehydrogenase when glucose is the target substance; lactic acid oxidase or lactic acid dehydrogenase when lactic acid is the target substance; cholesterol esterase or cholesterol oxidase when cholesterol is the target substance; alcohol oxidase when alcohol is the target substance; and bilirubin oxidase when bilirubin is the target substance. The enzyme is not limited to these, however, and is appropriately selected according to the target substance. Other examples besides these of the target substance include triglyceride, uric acid, and so forth.
At least one compound selected from the group consisting of potassium ferricyanide, p-benzoquinone, p-benzoquinone derivatives, oxidizing phenazine methosulfate, methylene blue, ferricinium, and ferricinium derivatives can be used favorably, for example, as an electron mediator.
The spacer 13 is a plate-shaped member having a substantially rectangular cut-out 131. In a state in which the various members of the biosensor 10 have been assembled, the spacer 13 forms a sample supply path 10b surrounded by the substrate 11, the cover 14, and the inner faces of the cut-out 131. One end of the sample supply path 10b serves as the sample introduction port 10a.
The cover 14 is a flat, plate-shaped member. The cover 14 has an air hole 14a that passes through from the face opposite the spacer 13 to the face on the other side. The air hole 14a is disposed above the sample supply path 10b (discussed below), and in particular above the opposite end of the sample supply path 10b from the side with the sample introduction port 10a.
When the biosensor 10 is used, the user deposits a liquid sample in the sample introduction port 10a. The liquid sample deposited in the sample introduction port 10a moves through the sample supply path 10b into the biosensor 10 by capillary action, reaching the second working electrode 11b and the counter electrode 11d, and then reaching the first working electrode 11a and the detecting electrode 11c.
When the measurement system 100 is used to measure a blood glucose level, for example, the user pricks his own finger, hand, arm, etc., squeezes out a small quantity of blood, and supplies this blood as the liquid sample for measurement.
The constituent elements of the measurement device 20 will be described through reference to the block diagram of
As shown in
As shown in
As shown in
The switching circuit 202 is designed to switch the electrode connected to the reference voltage source 205, or to switch the voltage value that is applied to the electrodes.
The current/voltage conversion circuit 203 converts the amount of current flowing between the two electrodes connected to the current/voltage conversion circuit 203 into a voltage value when a signal directing that a current value be acquired is provided from the control circuit. After this, the converted voltage value is converted into a digital value by the A/D conversion circuit 204, inputted to a CPU 206a, and stored in the memory of the computer 206.
The computer 206 has the CPU (central processing unit) 206a and a memory 206b. The memory holds a conversion table Tb1 used to determine the concentration of the target substance in a blood sample, and a correction amount table Tb2 used to determine the correction amount for this concentration.
The computer 206 calculates the tentative concentration of the target substance by referring to the conversion table Tb1, on the basis of the above-mentioned stored voltage value. The computer 206 determines the ultimate concentration of the target substance by using a correction amount found in the correction amount table Tb2. This processing will be discussed in greater detail below.
In addition to its functions as concentration acquisition component and correction component, the computer 206 also has the functions of controlling the switching of the switching circuit 202, inputting from the A/D conversion circuit 204, controlling the voltage of the reference voltage source 205 serving as the voltage application component, measuring the length of time during concentration measurement or correction amount selection (timer function), outputting display data to the display component 21, and communicating with external devices. The computer 206 also exercises controls over the entire measurement device 20.
The various functions of the computer 206 are realized by having the CPU 206a read and execute programs stored in a ROM (read only memory; not shown) or the like.
When the biosensor 10 is inserted into the insertion component 22 of the measurement device 20, the switching circuit 202 receives a command from the CPU 206a of the computer 206 to connect the first working electrode 11a to the reference voltage source 205 via the connector 201a, and to connect the detecting electrode 11c to the current/voltage conversion circuit 203 via the connector 201c. In this connection state, when a liquid sample is deposited in the sample introduction port 10a of the biosensor 10, this liquid sample moves through the sample supply path 10b under capillary action, and reaches the detecting electrode 11c, there is a change in the current value between the first working electrode 11a and the detecting electrode 11c. As a result, the voltage value inputted to the computer 206 changes (increases). This change is detected by the CPU 206a. This detection triggers the following operations (a) and (b).
(a) Detection of Interfering Substances
As shown in
The times t1 to t4 are not limited to any specific numerical values, so long as they satisfy the relation t1<t2<t3<t4. The times t1 to t4 can be suitable set according to the length of the sample supply path 10b (the length of the biosensor 10 in the lengthwise direction), the width of the sample supply path 10b (the length of the biosensor 10 in the lateral direction), the composition of the reagent layer 12, the ease of dissolving the reagent layer 12 in the liquid sample, and so on.
Up until the counting of the time t4 ends, the switching circuit 202 maintains the connection of the second working electrode 11b and the reference voltage source 205, as well as the connection of the counter electrode 11d and the current/voltage conversion circuit 203, at a command from the CPU 206a.
Until the time t1 has elapsed from the start of the count (time 0 (zero)), that is, in the first step S1, the voltage applied between the second working electrode 11b and the counter electrode 11d is kept at an open circuit voltage at a command from the CPU 206a.
After the end of counting the time t1, until the end of the counting of the time t2, that is, in the second step S2 (second voltage application step), a closed circuit voltage v1 (second voltage) is applied between the second working electrode 11b and the counter electrode 11d.
After the end of counting the time t2, until the end of the counting of the time t3, that is, in the third step S3, the voltage applied between the second working electrode 11b and the counter electrode 11d is kept at an open circuit voltage.
After the end of counting the time t3, until the end of the counting of the time t4, that is, in the fourth step S4 (second voltage application step), a closed circuit voltage v2 (second voltage) is applied between the second working electrode 11b and the counter electrode 11d.
The times t1 to t4 are preferably set so that the voltage application of the second step S2 and the fourth step S4 is carried out before the redox reaction of the target substance in the liquid sample has an effect, that is, before the dissolved reagent layer reaches the second working electrode.
More specifically, the length of the first step S1 (that is, the time t1) may be set to fall within a range of at least 0 seconds and no more than 1 second, the length of the second step S2 (that is, the time (t2−t1)) may be set to fall within a range of at least 0.5 second and no more than 1.0 second, the length of the third step S3 (that is, the time (t3−t2)) may be set to fall within a range of at least 0.5 second and no more than 3.0 seconds, and the length of the fourth step S4 (that is, the time (t4−t3)) may be set to fall within a range of at least 1.0 second and no more than 3.0 seconds
The voltages v1 and v2 are preferably at least the potential at which the redox reaction of the various interfering substances is carried out. Also, the voltages v1 and v2 are preferably set lower than the value at which electrolysis of water will occur. Consequently, an accurate measurement result can be obtained for interfering substances, without the liquid sample itself being electrolyzed. The voltage value at which electrolysis occurs will vary with the biosensor electrode structure and so forth, but is about 2.0 V, for example.
Also, the voltage v1 is preferably lower than the voltage v2. This is because the redox potential varies with the type of interfering substance. More specifically, the voltage v1 is preferably set to a value such that a redox reaction will occur with a certain interfering substance, but is not likely to occur with another interfering substance. The voltage v2 is set to a value at which a redox reaction will occur with both interfering substances. Setting the voltages v1 and v2 in this way means that the amounts of change (that is, the slope) in the current value (discussed below) will have mutually different values depending on the kind of interfering substance, so it is possible to correct the measured value for concentration of a target substance according to the type of interfering substance.
Thus, the voltage v1 is preferably set within a range of 0.1V≦v1≦1.0 V, and more preferably within a range of 0.1 V≦v1≦0.3 V.
Also, the voltage v2 is preferably set within a range of 0.1 V≦v2≦1.0 V, and more preferably within a range of 0.4 V≦v2≦0.8 V.
Curves A (solid line), B (fine dotted line), and C (thick dotted line) in
In all of the measurements, FAD-GDH (glucose dehydrogenase in which flavin adenine dinucleotide serves as a coenzyme; (hereinafter referred to as FAD-GDH)) was used as the oxidoreductase. Also, the first step S1 lasted 1 second, the second step S2 lasted 0.5 second, the third step S3 lasted 1 second, and the fourth step S4 lasted 1 second, so the current value measurement was conducted for a total of 3.5 seconds. The voltage v1 was 0.3 V, and the voltage v2 was 0.6 V.
As shown in
The CPU 206a of the computer 206 calculates the slope from the maximum current value in the second step S2 up to the maximum current value in the fourth step S4, among the current values obtained for one kind of sample.
More specifically, as shown in
Thus, the computer 206 can reduce the error in concentration correction by calculating the slope between maximum response current values for the various applied voltage values.
As discussed below, the current values at the end of the counting of the time t4 (the final response values ia2, ib2, and ic2) can also be used for concentration correction.
These slopes and final response values are stored in the memory 206b of the computer 206.
The term “slope” here is the increase in the vertical axis component (current value) divided by the increase in the horizontal axis component (time), so it is the increase in current value per unit of time.
Also, as discussed above, the value obtained by converting a current value into a voltage value is what is actually used by the computer 206 to calculate the slopes a to c. However, since a voltage value obtained by conversion reflects the level of the current, we have used the term “current value” here for the sake of convenience.
(b) Detection of Target Substance
As discussed above, the following operation triggers a change in current value between the first working electrode 11a and the detecting electrode 11c. Specifically, the detection of the target substance is executed in parallel with the detection of an interfering substance discussed in section (a) above. The start timing of the target substance detection is not limited to this.
When it is detected that a liquid sample has reached the detecting electrode 11c, the CPU 206a outputs a command for the switching circuit 202 to connect the first working electrode 11a and the reference voltage source 205, and to connect the counter electrode 11d and the current/voltage conversion circuit 203, and the detection of the target substance (such as glucose) is performed.
When the liquid sample reaches the reagent layer 12, the reagent layer 12 dissolves in the liquid sample, which forms a reaction sample that includes the liquid sample and the oxidoreductase, and oxidants or reductants of the electron mediator gradually begin to be produced by the redox reaction within the reaction sample.
The CPU 206a can detect the concentration of these oxidants or reductants, that is, the concentration of the target substance, by detecting a change in the current value between the first working electrode 11a and the counter electrode 11d.
As mentioned above, the point at which a change in current value between the first working electrode 11a and the detecting electrode 11c was detected is set to 0 (zero), and the CPU 206a of the computer 206 begins counting the times T1, T2, and T3.
There are no specific limits on the numerical values of the times T1 to T3, so long as they satisfy the relation T1<T2<T3, and they are values that give a current value satisfactory for measuring the concentration of the target substance. The times T1 to T3 are set by taking into account the type of target substance, the configuration of the biosensor 10, the composition of the reagent layer 12, and other such conditions.
As shown in
After the end of the counting of the time T1, until the end of the counting of the time T2, that is, in the sixth step S6, the voltage applied between the first working electrode 11a and the counter electrode 11d is open circuit voltage.
After the end of the counting of the time T2, until the end of the counting of the time T3, that is, in the seventh step S7 (first voltage application step), a closed circuit voltage Vb (first voltage) is applied between the first working electrode 11a and the counter electrode 11d.
Here, the application of voltage in the fifth step S5 is preprocessing of the liquid sample. A current value obtained in the seventh step S7 is actually used in the measurement of the concentration of the target substance. Usually, the voltage Va is set to be higher than Vb. The voltage Vb is set so that reducing electron transporters produced by the enzyme reaction can be oxidized, or so that oxidizing electron transporters produced by the enzyme reaction can be reduced.
In the example shown in
Curves A, B, and C in
(c) Concentration Calculation
The CPU 206a selects as a tentative concentration the concentration corresponding to the current value at T3 obtained in the processing of section (b) above, from the conversion table Tb1, on the basis of this current value (concentration acquisition step).
In the example in
As shown in curves A and B of
(d) Correction
The CPU 206a selects a correction amount from the correction amount table Tb2 on the basis of the value of the slope obtained in the processing of section (a), uses this correction amount to correct the concentration obtained in the processing of section (c), and uses this result as the final measurement result.
The CPU 206a selects a correction amount on the basis of the slope value obtained in the processing of section (a), the final response value obtained in the processing of section (a) (Int (μA)), and the tentative concentration obtained in the processing of section (c) (correction amount selection step).
The CPU 206a corrects the measurement result for the concentration of the target substance in (c) above on the basis of the selected correction amount (correction step).
The processing in the examples in
First, in the example in
As shown in
Next, a case performing the processing of this embodiment will be described.
First, sample B, in which the interfering substance is acetaminophen, will be described. In the processing of section (a) above, as shown in
In the fourth step S4, however, when a high voltage of 0.6 V was applied, the peak value Ib2 exhibited a high value of 4.1 μA. The slope b found by subjecting these two peak values to linear regression was 2.6. The final response value ib2 was 1.38 μA.
The glucose concentration calculated in the processing of (c) above was 108 mg/dL, so a correction amount was selected that corresponded to a glucose concentration of 100 mg/dL, a slope of ≧2.4, and an Int of 1.38 μA. As shown in
As shown in
Next, sample C, in which the interfering substance is ascorbic acid, will be described.
In the processing of section (a) above, as shown in
The glucose concentration calculated in the processing of (c) above was 118 mg/dL, so a correction amount was selected that corresponded to a glucose concentration of 100 mg/dL, a slope of ≦1.5, and an Int of 1.65 μA. As shown in
(2-1) With the measurement device 20 in the first embodiment, in the processing of (a) above, it is possible to measure the concentration accurately by measuring the current values under voltage values of two different levels, and determining the correction amount with respect to an interfering substance from the slope between these current values.
However, the present invention is not limited to this, and a correction amount may be selected on the basis of the slope obtained by obtaining current values under three or more different voltage values and subjecting these three or more current values to linear regression.
The more variety of voltage that is applied, the greater will be the accuracy.
(2-2) If three or more current values are obtained, the “slope” used in determining the correction amount can also be obtained by selecting some of these current values and subjecting them to linear regression.
(2-3) The correction amount can also be selected on the basis of the amount of change in the current value other than slope. For instance, current values may be measured under two voltage values, the difference between the two resulting current values calculated by the CPU, and the correction amount selected by the CPU on the basis of this difference.
(2-4) In the first embodiment, in the second step S2 and the fourth step S4, the voltage value was assumed to be constant within a single step. That is, in the first embodiment the second step S2 and the fourth step S4 were second voltage application steps of applying a second voltage, for obtaining a current value used in determining the correction amount, to a second electrode system (the second working electrode 11b and the counter electrode 11d). In other words, with the measurement device 20 in the first embodiment, in the second step S2 and the fourth step S4, a second voltage application operation is executed. In the first embodiment, the voltage v1 is applied as the second voltage, after the application of the voltage v1, the voltage v2 that is higher than the voltage v1 is applied.
However, the second voltage application operation, the second voltage application step, and the second voltage are not limited to the above. For example, the configuration may be such that the computer 206 sweeps the voltage applied to the second working electrode 11b from the reference voltage source 205. Specifically, the computer 206 may continuously vary the value of the second voltage. In particular, it is preferable if the voltage value is controlled so as to increase steadily with respect to the elapsed time. When sweeping is performed, the voltage values in the various steps are the same as discussed above.
More specifically, as shown in
In
In this embodiment, the flow of detecting the target substance can be the same as in the first embodiment.
(2-5) Also, as discussed above, the correction amount is preferably determined from the amount of change between the peak current values, but the amount of change may be obtained on the basis of the current values at timings other than the peaks.
(2-6) Besides selecting from a table, the method for determining the correction amount and the tentative concentration can be to calculate it from a calibration curve or another such method.
(2-7) In the first embodiment, the first electrode system and second electrode system shared a single counter electrode 11d. However, the first electrode system and second electrode system may instead each have its own separate counter electrode.
(2-8) In the first embodiment, the reagent layer 12 is disposed so as not to be over the second working electrode 11b. However, a reagent layer that is different from the reagent layer used to detect the target substance may be disposed over the second working electrode. This different reagent layer may be referred to instead as “a reagent layer having a different composition,” and examples of such a reagent layer include a reagent layer that does not contain an enzyme.
(2-9) In the first embodiment, the fifth step S5 and the seventh step S7 were executed as the first voltage application step. In other words, with the measurement device 20 of the first embodiment, in the fifth step S5 and the seventh step S7, a first voltage application operation is executed. The voltage Va is applied as the first voltage in the fifth step S5, and the voltage Vb that is lower than the voltage Va is applied in the subsequent seventh step S7. The concentration of the target substance is measured on the basis of the current value at the application of the voltage Vb.
However, the first voltage application operation, the first voltage application step, and the first voltage are not limited to the above, and can be modified to the extent that the concentration of the target substance can still be measured, according to the type of target substance, the structure of the biosensor, and other conditions.
A measurement system pertaining to a comparative embodiment will now be described. Those members having the same function as the members already described will be numbered the same and not described again.
As shown in the exploded oblique view of
The measurement system of this embodiment also differs in the measurement operation of the measurement device (not shown).
The CPU then obtains a tentative concentration by referring to a conversion table, on the basis of the current value in
In this comparative embodiment, the proportions in which the acetaminophen-containing sample and the ascorbic acid-containing sample are contained in the liquid sample cannot be specified. Accordingly, correction had to be performed at the same correction amount, for which the proportions of the various interfering substances contained in the liquid sample was estimated ahead of time.
That is, even though the effect on the detection result for the target substance varies with the type of interfering substance in this comparative embodiment, correction is performed with the same correction amount, so as shown in
In contrast, in the working example given above, although the same table was used for both acetaminophen and ascorbic acid, the correction amount is selected on the basis of the slope of the current values, so a different correction amount can be selected according to the interfering substance, and this allows a more appropriate correction to be performed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-082814 | Mar 2008 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/001427 | 3/27/2009 | WO | 00 | 7/29/2010 |
