CA2170660C - Method and apparatus for reduction of bias in amperometric sensors - Google Patents
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- C—CHEMISTRY; METALLURGY
- C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
- C12Q—MEASURING OR TESTING PROCESSES INVOLVING ENZYMES, NUCLEIC ACIDS OR MICROORGANISMS; COMPOSITIONS OR TEST PAPERS THEREFOR; PROCESSES OF PREPARING SUCH COMPOSITIONS; CONDITION-RESPONSIVE CONTROL IN MICROBIOLOGICAL OR ENZYMOLOGICAL PROCESSES
- C12Q1/00—Measuring or testing processes involving enzymes, nucleic acids or microorganisms; Compositions therefor; Processes of preparing such compositions
- C12Q1/001—Enzyme electrodes
- C12Q1/004—Enzyme electrodes mediator-assisted
Abstract
Apparatus and method are provided for determining the concentration of an analyte in a fluid test sample by applying the fluid test sample to the surface of a work- ing electrode which is electrochemically connected to a reference electrode which surface bears a composition comprising an enzyme specific for the analyte. A media- tor is reduced in response to a reaction between the ana- lyte and the enzyme. An oxidizing potential is applied between the electrodes to return at least a portion of the mediator back to its oxidized form before determining the concentration of the analyte to thereby increase the accuracy of the analyte determination. Following this initially applied potential, the circuit is switched to an open circuit or to a potential that substantially re- duces the current to minimize the rate of electrochemical potential at the working electrode. A second potential is applied between the electrodes and the current gener- ated in the fluid test sample is measured to determine analyte concentration. Optionally, the accuracy of the analyte determination is further enhanced algorithmi- cally.
Description
METHOD AND APPARATUS FOR REDUCTION OF BIAS IN
AMPEROMETRIC SENSORS
Field of the Invention The present invention generally relates to a biosen-sor, and, more particularly, to a new and improved method and apparatus for reducing bias in amperometric sensors.
Background of the Invention The quantitative determination of analytes in body fluids is of great importance in the diagnoses and main-tenance of certain physiological abnormalities. For ex-ample lactate, cholesterol and bilirubin should be moni-tored in certain individuals. In particular, the deter-mination of glucose in body fluids is of great importance to diabetic individuals who must frequently check the level of glucose in their body fluids as a means of regu-lating the glucose intake in their diets. While the re-mainder of the disclosure herein will be directed towards the determination of glucose, it is to be understood that the procedure and apparatus of this invention can be used for the determination of other analytes upon selection of the appropriate enzyme. The ideal diagnostic device for the detection of glucose in fluids must be simple, so as not to require a high degree of technical skill on the part of the technician administering the test. In many cases, these tests are administered by the patient which lends further emphasis to the need for a test which is easy to carry out. Additionally, such a device should be based upon elements which are sufficiently stable to meet situations of prolonged storage.
MSE #1899
AMPEROMETRIC SENSORS
Field of the Invention The present invention generally relates to a biosen-sor, and, more particularly, to a new and improved method and apparatus for reducing bias in amperometric sensors.
Background of the Invention The quantitative determination of analytes in body fluids is of great importance in the diagnoses and main-tenance of certain physiological abnormalities. For ex-ample lactate, cholesterol and bilirubin should be moni-tored in certain individuals. In particular, the deter-mination of glucose in body fluids is of great importance to diabetic individuals who must frequently check the level of glucose in their body fluids as a means of regu-lating the glucose intake in their diets. While the re-mainder of the disclosure herein will be directed towards the determination of glucose, it is to be understood that the procedure and apparatus of this invention can be used for the determination of other analytes upon selection of the appropriate enzyme. The ideal diagnostic device for the detection of glucose in fluids must be simple, so as not to require a high degree of technical skill on the part of the technician administering the test. In many cases, these tests are administered by the patient which lends further emphasis to the need for a test which is easy to carry out. Additionally, such a device should be based upon elements which are sufficiently stable to meet situations of prolonged storage.
MSE #1899
2~ 70~~0 Methods for determining analyte concentration in fluids can be based on the electrochemical reaction be-tween the analyte and an enzyme specific to the analyte and a mediator which maintains the enzyme in its initial oxidation state. Suitable redox enzymes include oxi dases, dehydrogenases, catalase and peroxidase. For ex ample, in the case where glucose is the analyte, the re action with glucose oxidase and oxygen is represented by equation (A).
glucose + OZ glucose oxidase (GO) ~ gluconolactone + HZO2 (A) In a colorimetric assay, the released hydrogen per-oxide, in the presence of a peroxidase, causes a color change in a redox indicator which color change is propor-tional to the level of glucose in the test fluid. While colorimetric tests can be made semi-quantitative by the use of color charts for comparison of the color change of the redox indicator with the color change obtained using test fluids of known glucose concentration, and can be rendered more highly quantitative by reading the result with a spectrophotometric instrument, the results are generally not as accurate nor are they obtained as quickly as those obtained using a biosensor. As used herein, the term biosensor is intended to refer to an analytical device that responds selectively to analytes in an appropriate sample and converts their concentration into an electrical signal via a combination of a biologi-cal recognition signal and a physico-chemical transducer.
Aside from its greater accuracy, a biosensor is an in-MSE #1899
glucose + OZ glucose oxidase (GO) ~ gluconolactone + HZO2 (A) In a colorimetric assay, the released hydrogen per-oxide, in the presence of a peroxidase, causes a color change in a redox indicator which color change is propor-tional to the level of glucose in the test fluid. While colorimetric tests can be made semi-quantitative by the use of color charts for comparison of the color change of the redox indicator with the color change obtained using test fluids of known glucose concentration, and can be rendered more highly quantitative by reading the result with a spectrophotometric instrument, the results are generally not as accurate nor are they obtained as quickly as those obtained using a biosensor. As used herein, the term biosensor is intended to refer to an analytical device that responds selectively to analytes in an appropriate sample and converts their concentration into an electrical signal via a combination of a biologi-cal recognition signal and a physico-chemical transducer.
Aside from its greater accuracy, a biosensor is an in-MSE #1899
3 strument which generates an electrical signal directly thereby facilitating a simplified design. In principle, all the biosensor needs to do is measure the time and read the current. Furthermore, a biosensor offers the advantage of low material cost since a thin layer of chemicals is deposited on the electrodes and little mate-rial is wasted.
Referring to the above equation (A), a suitable electrode can measure the formation H20z due to its in-troduction of electrons into the test fluid according to equation B:
HZQZ >0Z + 2H+ + 2e-(B) The electron flow is then converted to the electrical signal which directly correlates to the glucose concen-tration.
In the initial step of the reaction represented by equation (A), glucose present in the test sample converts the oxidized flavin adenine dinucleotide (FAD) center of the enzyme into its reduced form, (FADHZ). Because these redox centers are essentially electrically insulated within the enzyme molecule, direct electron transfer to the surface of a conventional electrode does not occur to any measurable degree in the absence of an unacceptably high cell voltage. An improvement to this system in-wolves the use of a nonphysiological redox coupling be-tween the electrode and the enzyme to shuttle electrons between the (FADHz) and the electrode. This is repre-MSE #1899 ' 4 sented by the following scheme in which the redox cou-pler, typically referred to as a mediator, is represented by M:
Glucose + GO(FAD) -> gluconolactone + GO(FADHz) GO ( FADHZ ) + 2M~ > GO ( FAD ) + 2Mr~ + 2H~
2Mr~ > 2M~ + 2e (at the electrode) In the scheme, GO(FAD) represents the oxidized form of glucose oxidase and GO(FADHZ) indicates its reduced form. The mediating species MoX/Mred shuttles electrons from the reduced enzyme to the electrode thereby oxidiz-ing the enzyme causing its regeneration in situ which, of course, is desirable for reasons of economy. The main purpose for using a mediator is to reduce the working po tential of the sensor. An ideal mediator would be re oxidized at the electrode at a low potential under which impurity in the chemical layer and interfering substances in the sample would not be oxidized thereby minimizing interference.
Many compounds are useful as mediators due to their ability to accept electrons from the reduced enzyme and transfer them to the electrode. Among the mediators known to be useful as electron transfer agents in ana-lytical determinations are the substituted benzo- and naphthoquinones disclosed in U.S. Patent 4,746,607; the N-oxides, nitroso compounds, hydroxylamines and oxines specifically disclosed in EP 0 354 441; the flavins, phenazines, phenothiazines, indophenols, substituted 1,4-benzoquinones and indamins disclosed in EP 0 330 517 and the phenazinium/phenoxazinium salts described in U.S.
MSE #1899 Patent 3,791,988. A comprehensive review of electro-chemical mediators of biological redox systems can be found in Analytica Clinica Acta. 140 (1982), Pp 1-18.
5 Among the more venerable mediators is hexacyanofer-rate, also known as ferricyanide, which is discussed by Schlapfer et al in Clinica Chimica Acta., 57 (1974), Pp.
283-289. In U.S. Patent 4,929,545 there is disclosed the use of a soluble ferricyanide compound in combination with a soluble ferric compound in a composition for enzy-maticalTy determining an analyte in a sample. Substitut-ing the iron salt of ferricyanide for oxygen in equation (A) provides:
Glucose + 2 Fe+++ ( CN ) 3 6 ~° > gluconolactone + 2 Fe++ ( CN ) ° 6 since the ferricyanide is reduced to ferrocyanide by its acceptance of electrons from the glucose oxidase enzyme.
Another way of expressing this reaction is by use of the following equation (C):
Glucose + GO(FAD) > Gluconalactone + GO(FADHZ) GO(FADHZ) + 2 Fe(CN3)3 6 -> GO(FAD) + 2 Fe(CN)6° + 2H+
2 5 2 Fe(CN)64 > 2 Fe(CN)63 + 2e (at the electrode) (C) The electrons released are directly equivalent to the amount of glucose in the test fluid and can be related thereto by measurement of the current which is produced through the fluid upon the application of a potential MSE #1899 2170bb0 thereto. Oxidation of the ferrocyanide at the anode re-news the cycle.
As is apparent from the above description, a neces-sary attribute of a mediator is the ability to remain in the oxidized state under the conditions present on the electrode surface prior to the use of the sensor. Any reduction of the mediator will increase the background current resulting in the biosensor reading being biased.
It has been discovered that these mediators do tend to be reduced over time, especially under conditions of stress, thereby diminishing the usefulness of the sensors to which they are applied.
In published international patent application PCT/US92/01659 there is disclosed the use of potassium dichromate as an oxidizing agent in a colorimetric rea-gent strip. The purpose of the oxidizing agent is to oxidize impurities in other reagent components to improve the colorimetric sensor's stability. This publication mentions USSN 07/451,671 (now U.S. 5,288,636) and charac-terizes it as describing a system in which a reduced me-diator is re-oxidized by the application of a potential and measuring the current after a specific time to deter-mine the concentration of the analyte. More specifi-cally, the '636 patent requires the complete oxidation of the glucose by glucose oxidase. As the enzyme is reduced by the glucose, the ferricyanide reacts with enzyme to produce ferrocyanide. The ferrocyanide produced by this enzymatic reaction is combined with ferrocyanide produced during storage. This latter ferrocyanide is the result of a reaction between ferricyanide and impurities found MSE #1899 in materials deposited with the glucose oxidase and fer-ricyanide. The '636 patent makes no distinction between ferrocyanide produced between these two sources.
It would be desirable, and it is an object of the present invention to provide a method whereby the unde sired reduction of mediator compounds stored on an elec trodes surface can be reversed to minimize its effect on estimating the analyte values in fluid test samples with very low analyte concentrations.
It is a further object to provide such a method in which the accuracy of the analyte determination is en-hanced.
It is a further object to provide such a method wherein the analyte is glucose.
An additional object is to provide a mathematical means for further enhancement of the accuracy of the ana lyte determination.
It is a further object to provide apparatus for ac-curately determining analyte values.
It is a further object to provide such apparatus that is simple and economical to manufacture.
MSE #1899 217Jb60 Summary of the Invention The present invention involves a method for deter-mining the concentration of an analyte in a fluid test sample by applying the test sample to the surface of a working electrode. The electrode has on its surface a composition comprising an enzyme specific for the ana-lyte, a mediator which is reduced as a result of a reac-tion between the analyte and the enzyme, which mediator has undergone partial reduction to its reduced state as a result of having been exposed to ambient conditions.
There is disclosed herein an improvement to the method which involves the steps of:
a) applying a positive potential pulse to the electrode to oxidize at least a portion of the me-diator to its oxidized form. This step reduces background bias in the electrode. The background bias can be further reduced by:
a) determining the current (i1) during the ap-plication of the positive pulse and the current (i2) at the end of the read time, and b) calculating the corrected analyte level G
by solving equation (1):
_ i, - Int _ G s~pe k ~ ~~5'h ~ ( 1 ) where Int and slope are the intercept and slope of i2 and 0(i1, i2) is an error correction term MSE #1899 - 217~bbU
proportional to the background bias calculated as:
~~~ ~ ) = Int _ slops ~ i, - s, ~ is -1 ( 2 slopc slops ~ i,_,~ - s, ~ i=-~
where s1 = slope of i1 ii-to = i1 at a low analyte level, iz_to = iz at a low analyte level, and k = a selected scaling factor.
Brief Description of the Drawings The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the preferred em bodiments of the invention illustrated in the drawings, wherein:
J
FIG. 1 is a chart illustrating potential and current relative to time in accordance with the method of the in-vention;
FIG. 2 is a block diagram representation of a device for determining analyte values employed to perform the method of the invention; and FIG. 3 is a flow chart illustrating the sequential steps performed by a processor of FIG. 2 in accordance with the method of the invention.
MSE #1899 Description of the Invention The present invention is a method that reduces the 5 background bias due to oxidizable impurities in an am-perometric sensor used for measuring a specific analyte, such as glucose, in blood. The background current of such a sensor will increase if it is stored over a long period of time or under stress (heat, moisture, etc.) 10 due to the increased presence of reduced mediator or other reduced impurity present in the sensor such as en-zyme stabilizers, e.g. glutamate, and surfactants having reducing equivalents. For example, in a ferricyanide based amperometric sensor, the background bias is related to the presence of ferrocyanide (from the reduction of ferricyanide) near the electrode surface. This accumu-lated ferrocyanide, as opposed to the ferrocyanide pro-duced during use of the sensor (fresh ferrocyanide), is oxidized back to ferricyanide to reduce the background bias it causes and thereby extend the sensor shelf life.
To achieve this objective, the method uses an electro-chemical approach. The background bias is further re-duced when the electrochemical approach is augmented with an algorithmic correction.
Referring to FIG. 1, the method of our invention in-volves first applying a positive potential pulse (called the " burn-off " pulse) which precedes the normal poten-tial profile during use of the sensor. This is typically accomplished by applying a positive potential of from 0.1 to 0.9 volt (preferably 0.3 to 0.7 volt) between the working and reference electrodes of the sensor for a pe-MSE #1899 ~..-riod of from 1 to 15 seconds (preferably 5 to 10 sec-onds). The burn-off pulse oxidizes the initial ferrocya-nide (or other oxidizable impurity), so that the sensor can begin the assay with a clean background. Typically, the background is not perfectly clean since only a por-tion of the oxidizable impurity is oxidized by the burn-off pulse. This is the case because the chemical layer covers both the working and the reference electrodes.
The initial ferrocyanide exists in the chemical layer since it comes from ferricyanide. When sample fluid is applied and the chemical layer re-hydrates, the ferrocya-nide near the working electrode is re-oxidized. The rest of the ferrocyanide diffuses into the sample fluid and is mixed with the glucose. That portion of the initial fer-rocyanide cannot be re-oxidized without affecting the glucose. The initial ferrocyanide is near the electrode for a very short time (a few seconds) after the fluid test sample is applied. The reason for this is that the chemicals (enzyme and ferricyanide, etc.) are deposited , as a thin layer on the working and reference electrodes.
The burn-off technique takes advantage of this since a significant amount of the initial ferrocyanide can be burned off without noticeable reduction of the analyte concentration in the fluid test sample most of which does not come into direct contact with the electrode. Experi-ments have demonstrated that the background bias of a stressed sensor can be reduced by 40~ with proper appli-cation of the burn-off pulse.
The background bias can be further reduced by the use of a background correction algorithm which works in conjunction with the burn-off pulse. The algorithm is MSE #1899 217~6~~
based on the taking of two current readings. The first reading (i1) is taken during the burn-off pulse and the second (i2) at the end of the read time, i.e. the time elapsed from the moment when the second potential pulse is applied to the moment when the current i2 is measured.
The length of the read time is t3-tz, as shown in FIG. 1.
The analyte concentration is then calculated from the two current readings, ii and i2. Tests on sensors have shown that the background correction algorithm is able to re-move at least 80% of the remaining background bias, and, as a result, the sensor stability can be improved to pro-vide a significant extension in shelf life.
An amperometric glucose sensor of the type useful in the practice of the present invention is constructed as follows: Two carbon electrodes are printed on a polymer substrate. Next a layer of chemical components is depos-ited on the electrodes and dried. A preferred chemical composition is 5 NL of a medium containing 55 mM ferricy-anide (potassium salt), 8.5 units of glucose oxidase, 0.53% of polyethylene oxide), 0.40% of cremophor as sur-factant and 83 mM phosphate buffer at pH 7.2. During the glucose assay, a potential profile consisting of three consecutive time periods is applied to the sensor. These time periods are, in sequence, the burn-off time (typically 0.4 volt for 10 seconds); delay period (open circuitry for 15 seconds) and read time (0.4 volts, 5 seconds). The exact time of the delay period is not critical but is normally in the range of 10 to 40 sec-onds. This delay period allows sufficient time for the reaction to build up sufficient ferrocyanide to allow the current resulting from the reoxidation of the ferrocya-MSE #1899 217~~660 nide to be measured without difficulty. These time peri-ods are illustrated in FIG. 1 which plots potential and current against time. Current measurements are taken at the end of the burn-off period (i1) and read time (i2) whereupon the corresponding glucose concentration is cal-culated using equation 1. The constants in the equation, e.g. slopes and intercepts are predetermined values.
The following discussion relates to a fluid test sample in which glucose is the analyte to be detected and involves a sensor in which ferricyanide is the mediator.
However, the discussion is equally applicable to systems for the determination of other analytes and in which the oxidizable species is something other than ferrocyanide.
The burn-off technique, i.e. application of a posi-tive potential pulse to the electrode to oxidize at least a portion of the mediator back to its oxidized form, is illustrated by FIG. 1. In FIG. 1, in which the potential and current profiles are plotted, the timing is as fol-lows:
to - sample is detected, burnoff period begins. Sam-ple is'detected by inserting the sensor into the in-strument which causes the immediate application,of a 0.4 volt potential. The current is continuously checked to see if a larger than predetermined threshold (e. g. 250 nA) is measured. When a larger current than the threshold value is detected, a sam-ple has been detected to begin the burnoff time pe-riod.
MSE #1899 t1 - end of burn-off period and current 1l is meas-ured. The length of the burnoff period, tl-to, is usually 5 to 10 seconds. The potential is 0.4 volt at t1 but switches to an open circuit or to a poten-tial that substantially reduces the current to mini-mize the rate of electrochemical reaction at the working electrode for a set delay period after the burnoff period.
t2 - end of set delay period. The length of the wait period, tz-tl, is normally 10 to 40 seconds . A read potential of 0.4 volt is applied at t2.
t3 - end of read time when current 1z is measured.
The length of the read time, t3-t2, is 5 to 10 sec onds.
The burn-off pulse, 1.e. application of the 0.4 volt potential from to to t1, is designed to eliminate part of initial ferrocyanide (accumulated ferro) or other oxidi-' zable interferents in the enzyme layer.
The burn-off algorithm calculates glucose concentra tion from two current measurements 1l and 1z using equa tion 1:
G = i,-Int K~A( iii=) slope (1) where 3 0 A 1 1 ) __ Int slope ~ 1, - s, ~ ii ('' 1 slope ~ slope ~ 1, ~, - s, ' 1z n ( 2 ) MSE #1899 217~J6b0 Equation 1 is a partial correction algorithm which is intended to achieve a compromise between reducing stress-related background bias and preserving system pre-y cision. The basic scheme is to use i2 as a glucose read-ing ' G = IZ - int - slope where int and slope are the intercept and slope of i2 re-spectively. The term 0(il,i2) is the estimated background increase, due to stress or other causes, derived from the current i1 and i2. For fresh sensors, this term is close to zero. The parameter k is selectively provided or set to a value from 0 to 1. There will be no background cor-rection if k is set at zero. On the other hand a full correction can be achieved if k is 1. In the following examples k is set at 0.8 for partial correction because it has been found that the variation of i1 is larger than that of i2 when multiple sensors are tested under the same glucose concentration. Compared with the glucose value calculated from i2 alone, k = 0 in equation (1), the glu-cose value calculated from i1 and i2 jointly will be slightly lower. in precision (a larger standard deviation) and, of course, a much smaller background bias. The tradeoff between the precision and bias can be achieved by choosing the proper k value. If k - 0, there is no backg-round correction and i1 is not used. In this case, the highest precision can be obtained, but it is accompa-nied by a high background bias. If k = 1, the full back-ground correction is applied whereupon the lowest bias can be achieved but at the cost of precision. The k MSE #1899 value is set at 0.8 in the example to achieve a compro-mise between precision and bias.
The parameters in these equations are:
Int - intercept of read current iz,nA.
slope - slope of read current iz,nA~dLlmg.
i~lo - average burn-off current il,nA, at the low glucose calibration level, i.e. 50 mg/dL.
izlo - average read time current iz,nA, at the low glucose calibration level. Actually, izlo is not an independent parameter. It can be cal-culated from Int and slope:
iz to = Int + slope ~ 50.
s1 - slope to burn-off current, nA~dL/mg.
k - set to 0.8 for partial correction.
Int, slope, illo, and s1 are local parameters; each sensor lot has its own parameter values which values are deter-mined experimentally. The algorithm needs two known cur-rent values, one for i1 and one for iz for normal (unstressed) sensors. The illo and izlo are available since they are used in determining the intercept (Int) and slopes (s1 and slope). Of course, current at other glucose levels can be used in the algorithm. This would however introduce the extra step of adding two additional independent parameters. The procedure of the present in - vention is demonstrated in the following examples:
MSE #1899 2~ 70660 Example I:
The following steps are taken to determine the lot parameter values necessary in the algorithm:
A. Test 16 sensors from the lot at the low cali bration level, 50 mgldL, and obtain the average cur rents i1 la and iz to of the burn-of f current and read time current, respectively. It is found that illo =
1951.2 nA and i2lo = 1952.3 nA.
B. Test 16 sensors at the high calibration level, 400 mgldL, and obtain the average current llhi and i2 n1~ It is found that 11 hi - 6003.3 nA and 12 hi 8831.7 nA.
C. Calculate the parameter values:
2 0 Int = Iz ~o - 50 - (iz ,~ - 1z iv ) =19523 - 50 - (8831.7 -19523) - 9695 nA
slope -- tz-''350z '~ - 8831. 350 952.3 =19.65 nA - dL l mg , s -1",, -1,_1o - 6003.3-19512 =1158 nA-dL l m 350 350 g Therefore, equation (1) becomes:
MSE #1899 G a i= -9695 19.65 -0.8~0(i,,h~
969.5 19.65~i, -11.58~iZ
19.65 ~ ( 19b5 ~ 1951.2 -11.58 ~ 19523 Vii'' h ~ _ -1) = 0.06162 ~ i, -0.03631 ~ i= -4934 Example II
It has been discovered that the burn-off pulse alone will significantly reduce the background bias even with-out the use of the background correction algorithm.
In this experiment, ten sensors were stressed under 30°C and 91% humidity for 3 hours. Aqueous glucose at 50 mg/dL was used as sample. Five stressed sensors were tested with a 10 second burn-off pulse and five without the pulse. In addition, ten unstressed sensors were tested as control (five with the 10 second burn-off and five without) and the bias calculated using the following equation (3):
_ bias = ~"'~'' t""~°' x 10096 ( 3 ) i,=",.., It was found that the bias was 30.6% without the burn-off pulse and 18.0% with it which data demonstrate that the burn-off pulse alone reduces the background bias by about 40%.
MSE #1899 21706b0 Example III
This example explains how the algorithm corrects for background bias:
Eight sensors were stored at below -20°C for two weeks and another eight sensors were stressed at 50°C for four weeks. All sixteen sensors were tested using whole blood having a 100 mg/dL glucose concentration. The pa-rameter values were determined from fresh sensors. The glucose readings, G, were calculated as follows:
A. No background bias correction algorithm: Equa-tion 1 with k = 0.
B. Partial correction: Equation 1 with k = 0.8.
The bias in percent is calculated using Equation 4 with the results being listed in Table 1.
bias = G-100 X 100 (4) 2 5 a tas at 100 m~ldL
no burn-off partial cornection al orithm -__ 0.8 -20 C, 2 weeks 3.8 % 5.3 ~
50 C, 4 weeks 64.7 % 15.0 %
Tabl 1 B' MSE #1899 217~~560 A device capable of carrying out the invention is represented by FIG. 2. Referring to FIG. 2, there is shown a block diagram representation of a device for ac-s curately determining analyte values designated as a whole by the reference character 10 and arranged in accordance with principles of the present invention. Device 10 in-cludes a microprocessor 12 together with a memory device 14. Microprocessor 12 is suitably programmed to perform 10 the method of the invention as illustrated in FIG. 3.
Various commercially available devices, such as a DS5000 microcontroller manufactured by Dallas Semiconductor, can be used for the microprocessor 12 and memory 14. Memory 14 can be included within the microprocessor 12 or sepa 15 rately provided as illustrated in FIG. 2.
Digital data from the microprocessor 12 is applied to a digital-to-analog (D/A) converter 16. D/A converter 16 converts the digital data to an analog signal. An am-20 plifier 18 coupled to the D/A converter 16 amplifies the analog signal. The amplified analog signal output of am-plifier 18 is applied to a sensor 20.
Sensor 20 is coupled to an amplifier 22. The ampli-fied sensed signal is applied to an analog-to-digital (A/D) converter 24 that converts the amplified, analog sensor signal to a digital signal. The digital signal is applied to the microprocessor 12.
Various commercially available devices can be used for D/A converter 16, amplifiers 18 and 20 and A/D con-verter 24. For example, a device type PM-752F4FS manu-factured by PMITcan be used for D/A converter 16. Opera tional amplifier device type TL074AC manufactured and sold by Linear Technology can be used for amplifiers 18 and 22. A device type MAX 135 CWI manufactured and sold 5 by MaxumTM can be used~for the A/D converter 24.
Referring also to FIG. 3, there are shown the se-quential steps for accurate analyte determination of the invention. Initially microprocessor 12 applies a burnoff 10 pulse, for example a potential of 0.4 volts, to the sen-sor 20 as indicated at a block 300. Then the microproc-essor checks to identify a sample corresponding to a de-tected sensor threshold current value as indicated at a decision block 302. When a sample is detected at block 15 302, a predetermined burnoff time interval, such as 10 seconds is identified at a decision block 304. Next the current i1 is measured as indicated at a block 306 and an open circuit is applied to the sensor 20 as indicated at a block 308. Then a set delay or predetermined wait time 20 interval, such as fifteen (15) seconds is identified at a decision block 310. After the set delay, a read pulse or potential of 0.4 volts is applied to the sensor 20 as in-dicated at a block 312. Then a predetermined read time interval for the read pulse, such as 5 seconds is identi-25 fied at a decision block 314 and the current i2 is meas-ured as indicated at a block 316. Next microprocessor 12 gets the stored parameters for a particular sensor 20 in-cluding Int, slope, i1 lo, iz lo, S1 and k, as indicated at a block 320. The correction term Delta (i1, i2) is calcu-30 lated utilizing the stored parameters and measured burn-off current i1 and read current i2 as indicated block 322.
Next the analyte value, such as glucose reading G, is 2i7~6bU
calculated utilizing the read current i2 and the calcu-lated correction term Delta (i1, i2) multiplied by the se-lected scaling value k, as indicated at a block 324.
MSE #1899
Referring to the above equation (A), a suitable electrode can measure the formation H20z due to its in-troduction of electrons into the test fluid according to equation B:
HZQZ >0Z + 2H+ + 2e-(B) The electron flow is then converted to the electrical signal which directly correlates to the glucose concen-tration.
In the initial step of the reaction represented by equation (A), glucose present in the test sample converts the oxidized flavin adenine dinucleotide (FAD) center of the enzyme into its reduced form, (FADHZ). Because these redox centers are essentially electrically insulated within the enzyme molecule, direct electron transfer to the surface of a conventional electrode does not occur to any measurable degree in the absence of an unacceptably high cell voltage. An improvement to this system in-wolves the use of a nonphysiological redox coupling be-tween the electrode and the enzyme to shuttle electrons between the (FADHz) and the electrode. This is repre-MSE #1899 ' 4 sented by the following scheme in which the redox cou-pler, typically referred to as a mediator, is represented by M:
Glucose + GO(FAD) -> gluconolactone + GO(FADHz) GO ( FADHZ ) + 2M~ > GO ( FAD ) + 2Mr~ + 2H~
2Mr~ > 2M~ + 2e (at the electrode) In the scheme, GO(FAD) represents the oxidized form of glucose oxidase and GO(FADHZ) indicates its reduced form. The mediating species MoX/Mred shuttles electrons from the reduced enzyme to the electrode thereby oxidiz-ing the enzyme causing its regeneration in situ which, of course, is desirable for reasons of economy. The main purpose for using a mediator is to reduce the working po tential of the sensor. An ideal mediator would be re oxidized at the electrode at a low potential under which impurity in the chemical layer and interfering substances in the sample would not be oxidized thereby minimizing interference.
Many compounds are useful as mediators due to their ability to accept electrons from the reduced enzyme and transfer them to the electrode. Among the mediators known to be useful as electron transfer agents in ana-lytical determinations are the substituted benzo- and naphthoquinones disclosed in U.S. Patent 4,746,607; the N-oxides, nitroso compounds, hydroxylamines and oxines specifically disclosed in EP 0 354 441; the flavins, phenazines, phenothiazines, indophenols, substituted 1,4-benzoquinones and indamins disclosed in EP 0 330 517 and the phenazinium/phenoxazinium salts described in U.S.
MSE #1899 Patent 3,791,988. A comprehensive review of electro-chemical mediators of biological redox systems can be found in Analytica Clinica Acta. 140 (1982), Pp 1-18.
5 Among the more venerable mediators is hexacyanofer-rate, also known as ferricyanide, which is discussed by Schlapfer et al in Clinica Chimica Acta., 57 (1974), Pp.
283-289. In U.S. Patent 4,929,545 there is disclosed the use of a soluble ferricyanide compound in combination with a soluble ferric compound in a composition for enzy-maticalTy determining an analyte in a sample. Substitut-ing the iron salt of ferricyanide for oxygen in equation (A) provides:
Glucose + 2 Fe+++ ( CN ) 3 6 ~° > gluconolactone + 2 Fe++ ( CN ) ° 6 since the ferricyanide is reduced to ferrocyanide by its acceptance of electrons from the glucose oxidase enzyme.
Another way of expressing this reaction is by use of the following equation (C):
Glucose + GO(FAD) > Gluconalactone + GO(FADHZ) GO(FADHZ) + 2 Fe(CN3)3 6 -> GO(FAD) + 2 Fe(CN)6° + 2H+
2 5 2 Fe(CN)64 > 2 Fe(CN)63 + 2e (at the electrode) (C) The electrons released are directly equivalent to the amount of glucose in the test fluid and can be related thereto by measurement of the current which is produced through the fluid upon the application of a potential MSE #1899 2170bb0 thereto. Oxidation of the ferrocyanide at the anode re-news the cycle.
As is apparent from the above description, a neces-sary attribute of a mediator is the ability to remain in the oxidized state under the conditions present on the electrode surface prior to the use of the sensor. Any reduction of the mediator will increase the background current resulting in the biosensor reading being biased.
It has been discovered that these mediators do tend to be reduced over time, especially under conditions of stress, thereby diminishing the usefulness of the sensors to which they are applied.
In published international patent application PCT/US92/01659 there is disclosed the use of potassium dichromate as an oxidizing agent in a colorimetric rea-gent strip. The purpose of the oxidizing agent is to oxidize impurities in other reagent components to improve the colorimetric sensor's stability. This publication mentions USSN 07/451,671 (now U.S. 5,288,636) and charac-terizes it as describing a system in which a reduced me-diator is re-oxidized by the application of a potential and measuring the current after a specific time to deter-mine the concentration of the analyte. More specifi-cally, the '636 patent requires the complete oxidation of the glucose by glucose oxidase. As the enzyme is reduced by the glucose, the ferricyanide reacts with enzyme to produce ferrocyanide. The ferrocyanide produced by this enzymatic reaction is combined with ferrocyanide produced during storage. This latter ferrocyanide is the result of a reaction between ferricyanide and impurities found MSE #1899 in materials deposited with the glucose oxidase and fer-ricyanide. The '636 patent makes no distinction between ferrocyanide produced between these two sources.
It would be desirable, and it is an object of the present invention to provide a method whereby the unde sired reduction of mediator compounds stored on an elec trodes surface can be reversed to minimize its effect on estimating the analyte values in fluid test samples with very low analyte concentrations.
It is a further object to provide such a method in which the accuracy of the analyte determination is en-hanced.
It is a further object to provide such a method wherein the analyte is glucose.
An additional object is to provide a mathematical means for further enhancement of the accuracy of the ana lyte determination.
It is a further object to provide apparatus for ac-curately determining analyte values.
It is a further object to provide such apparatus that is simple and economical to manufacture.
MSE #1899 217Jb60 Summary of the Invention The present invention involves a method for deter-mining the concentration of an analyte in a fluid test sample by applying the test sample to the surface of a working electrode. The electrode has on its surface a composition comprising an enzyme specific for the ana-lyte, a mediator which is reduced as a result of a reac-tion between the analyte and the enzyme, which mediator has undergone partial reduction to its reduced state as a result of having been exposed to ambient conditions.
There is disclosed herein an improvement to the method which involves the steps of:
a) applying a positive potential pulse to the electrode to oxidize at least a portion of the me-diator to its oxidized form. This step reduces background bias in the electrode. The background bias can be further reduced by:
a) determining the current (i1) during the ap-plication of the positive pulse and the current (i2) at the end of the read time, and b) calculating the corrected analyte level G
by solving equation (1):
_ i, - Int _ G s~pe k ~ ~~5'h ~ ( 1 ) where Int and slope are the intercept and slope of i2 and 0(i1, i2) is an error correction term MSE #1899 - 217~bbU
proportional to the background bias calculated as:
~~~ ~ ) = Int _ slops ~ i, - s, ~ is -1 ( 2 slopc slops ~ i,_,~ - s, ~ i=-~
where s1 = slope of i1 ii-to = i1 at a low analyte level, iz_to = iz at a low analyte level, and k = a selected scaling factor.
Brief Description of the Drawings The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the preferred em bodiments of the invention illustrated in the drawings, wherein:
J
FIG. 1 is a chart illustrating potential and current relative to time in accordance with the method of the in-vention;
FIG. 2 is a block diagram representation of a device for determining analyte values employed to perform the method of the invention; and FIG. 3 is a flow chart illustrating the sequential steps performed by a processor of FIG. 2 in accordance with the method of the invention.
MSE #1899 Description of the Invention The present invention is a method that reduces the 5 background bias due to oxidizable impurities in an am-perometric sensor used for measuring a specific analyte, such as glucose, in blood. The background current of such a sensor will increase if it is stored over a long period of time or under stress (heat, moisture, etc.) 10 due to the increased presence of reduced mediator or other reduced impurity present in the sensor such as en-zyme stabilizers, e.g. glutamate, and surfactants having reducing equivalents. For example, in a ferricyanide based amperometric sensor, the background bias is related to the presence of ferrocyanide (from the reduction of ferricyanide) near the electrode surface. This accumu-lated ferrocyanide, as opposed to the ferrocyanide pro-duced during use of the sensor (fresh ferrocyanide), is oxidized back to ferricyanide to reduce the background bias it causes and thereby extend the sensor shelf life.
To achieve this objective, the method uses an electro-chemical approach. The background bias is further re-duced when the electrochemical approach is augmented with an algorithmic correction.
Referring to FIG. 1, the method of our invention in-volves first applying a positive potential pulse (called the " burn-off " pulse) which precedes the normal poten-tial profile during use of the sensor. This is typically accomplished by applying a positive potential of from 0.1 to 0.9 volt (preferably 0.3 to 0.7 volt) between the working and reference electrodes of the sensor for a pe-MSE #1899 ~..-riod of from 1 to 15 seconds (preferably 5 to 10 sec-onds). The burn-off pulse oxidizes the initial ferrocya-nide (or other oxidizable impurity), so that the sensor can begin the assay with a clean background. Typically, the background is not perfectly clean since only a por-tion of the oxidizable impurity is oxidized by the burn-off pulse. This is the case because the chemical layer covers both the working and the reference electrodes.
The initial ferrocyanide exists in the chemical layer since it comes from ferricyanide. When sample fluid is applied and the chemical layer re-hydrates, the ferrocya-nide near the working electrode is re-oxidized. The rest of the ferrocyanide diffuses into the sample fluid and is mixed with the glucose. That portion of the initial fer-rocyanide cannot be re-oxidized without affecting the glucose. The initial ferrocyanide is near the electrode for a very short time (a few seconds) after the fluid test sample is applied. The reason for this is that the chemicals (enzyme and ferricyanide, etc.) are deposited , as a thin layer on the working and reference electrodes.
The burn-off technique takes advantage of this since a significant amount of the initial ferrocyanide can be burned off without noticeable reduction of the analyte concentration in the fluid test sample most of which does not come into direct contact with the electrode. Experi-ments have demonstrated that the background bias of a stressed sensor can be reduced by 40~ with proper appli-cation of the burn-off pulse.
The background bias can be further reduced by the use of a background correction algorithm which works in conjunction with the burn-off pulse. The algorithm is MSE #1899 217~6~~
based on the taking of two current readings. The first reading (i1) is taken during the burn-off pulse and the second (i2) at the end of the read time, i.e. the time elapsed from the moment when the second potential pulse is applied to the moment when the current i2 is measured.
The length of the read time is t3-tz, as shown in FIG. 1.
The analyte concentration is then calculated from the two current readings, ii and i2. Tests on sensors have shown that the background correction algorithm is able to re-move at least 80% of the remaining background bias, and, as a result, the sensor stability can be improved to pro-vide a significant extension in shelf life.
An amperometric glucose sensor of the type useful in the practice of the present invention is constructed as follows: Two carbon electrodes are printed on a polymer substrate. Next a layer of chemical components is depos-ited on the electrodes and dried. A preferred chemical composition is 5 NL of a medium containing 55 mM ferricy-anide (potassium salt), 8.5 units of glucose oxidase, 0.53% of polyethylene oxide), 0.40% of cremophor as sur-factant and 83 mM phosphate buffer at pH 7.2. During the glucose assay, a potential profile consisting of three consecutive time periods is applied to the sensor. These time periods are, in sequence, the burn-off time (typically 0.4 volt for 10 seconds); delay period (open circuitry for 15 seconds) and read time (0.4 volts, 5 seconds). The exact time of the delay period is not critical but is normally in the range of 10 to 40 sec-onds. This delay period allows sufficient time for the reaction to build up sufficient ferrocyanide to allow the current resulting from the reoxidation of the ferrocya-MSE #1899 217~~660 nide to be measured without difficulty. These time peri-ods are illustrated in FIG. 1 which plots potential and current against time. Current measurements are taken at the end of the burn-off period (i1) and read time (i2) whereupon the corresponding glucose concentration is cal-culated using equation 1. The constants in the equation, e.g. slopes and intercepts are predetermined values.
The following discussion relates to a fluid test sample in which glucose is the analyte to be detected and involves a sensor in which ferricyanide is the mediator.
However, the discussion is equally applicable to systems for the determination of other analytes and in which the oxidizable species is something other than ferrocyanide.
The burn-off technique, i.e. application of a posi-tive potential pulse to the electrode to oxidize at least a portion of the mediator back to its oxidized form, is illustrated by FIG. 1. In FIG. 1, in which the potential and current profiles are plotted, the timing is as fol-lows:
to - sample is detected, burnoff period begins. Sam-ple is'detected by inserting the sensor into the in-strument which causes the immediate application,of a 0.4 volt potential. The current is continuously checked to see if a larger than predetermined threshold (e. g. 250 nA) is measured. When a larger current than the threshold value is detected, a sam-ple has been detected to begin the burnoff time pe-riod.
MSE #1899 t1 - end of burn-off period and current 1l is meas-ured. The length of the burnoff period, tl-to, is usually 5 to 10 seconds. The potential is 0.4 volt at t1 but switches to an open circuit or to a poten-tial that substantially reduces the current to mini-mize the rate of electrochemical reaction at the working electrode for a set delay period after the burnoff period.
t2 - end of set delay period. The length of the wait period, tz-tl, is normally 10 to 40 seconds . A read potential of 0.4 volt is applied at t2.
t3 - end of read time when current 1z is measured.
The length of the read time, t3-t2, is 5 to 10 sec onds.
The burn-off pulse, 1.e. application of the 0.4 volt potential from to to t1, is designed to eliminate part of initial ferrocyanide (accumulated ferro) or other oxidi-' zable interferents in the enzyme layer.
The burn-off algorithm calculates glucose concentra tion from two current measurements 1l and 1z using equa tion 1:
G = i,-Int K~A( iii=) slope (1) where 3 0 A 1 1 ) __ Int slope ~ 1, - s, ~ ii ('' 1 slope ~ slope ~ 1, ~, - s, ' 1z n ( 2 ) MSE #1899 217~J6b0 Equation 1 is a partial correction algorithm which is intended to achieve a compromise between reducing stress-related background bias and preserving system pre-y cision. The basic scheme is to use i2 as a glucose read-ing ' G = IZ - int - slope where int and slope are the intercept and slope of i2 re-spectively. The term 0(il,i2) is the estimated background increase, due to stress or other causes, derived from the current i1 and i2. For fresh sensors, this term is close to zero. The parameter k is selectively provided or set to a value from 0 to 1. There will be no background cor-rection if k is set at zero. On the other hand a full correction can be achieved if k is 1. In the following examples k is set at 0.8 for partial correction because it has been found that the variation of i1 is larger than that of i2 when multiple sensors are tested under the same glucose concentration. Compared with the glucose value calculated from i2 alone, k = 0 in equation (1), the glu-cose value calculated from i1 and i2 jointly will be slightly lower. in precision (a larger standard deviation) and, of course, a much smaller background bias. The tradeoff between the precision and bias can be achieved by choosing the proper k value. If k - 0, there is no backg-round correction and i1 is not used. In this case, the highest precision can be obtained, but it is accompa-nied by a high background bias. If k = 1, the full back-ground correction is applied whereupon the lowest bias can be achieved but at the cost of precision. The k MSE #1899 value is set at 0.8 in the example to achieve a compro-mise between precision and bias.
The parameters in these equations are:
Int - intercept of read current iz,nA.
slope - slope of read current iz,nA~dLlmg.
i~lo - average burn-off current il,nA, at the low glucose calibration level, i.e. 50 mg/dL.
izlo - average read time current iz,nA, at the low glucose calibration level. Actually, izlo is not an independent parameter. It can be cal-culated from Int and slope:
iz to = Int + slope ~ 50.
s1 - slope to burn-off current, nA~dL/mg.
k - set to 0.8 for partial correction.
Int, slope, illo, and s1 are local parameters; each sensor lot has its own parameter values which values are deter-mined experimentally. The algorithm needs two known cur-rent values, one for i1 and one for iz for normal (unstressed) sensors. The illo and izlo are available since they are used in determining the intercept (Int) and slopes (s1 and slope). Of course, current at other glucose levels can be used in the algorithm. This would however introduce the extra step of adding two additional independent parameters. The procedure of the present in - vention is demonstrated in the following examples:
MSE #1899 2~ 70660 Example I:
The following steps are taken to determine the lot parameter values necessary in the algorithm:
A. Test 16 sensors from the lot at the low cali bration level, 50 mgldL, and obtain the average cur rents i1 la and iz to of the burn-of f current and read time current, respectively. It is found that illo =
1951.2 nA and i2lo = 1952.3 nA.
B. Test 16 sensors at the high calibration level, 400 mgldL, and obtain the average current llhi and i2 n1~ It is found that 11 hi - 6003.3 nA and 12 hi 8831.7 nA.
C. Calculate the parameter values:
2 0 Int = Iz ~o - 50 - (iz ,~ - 1z iv ) =19523 - 50 - (8831.7 -19523) - 9695 nA
slope -- tz-''350z '~ - 8831. 350 952.3 =19.65 nA - dL l mg , s -1",, -1,_1o - 6003.3-19512 =1158 nA-dL l m 350 350 g Therefore, equation (1) becomes:
MSE #1899 G a i= -9695 19.65 -0.8~0(i,,h~
969.5 19.65~i, -11.58~iZ
19.65 ~ ( 19b5 ~ 1951.2 -11.58 ~ 19523 Vii'' h ~ _ -1) = 0.06162 ~ i, -0.03631 ~ i= -4934 Example II
It has been discovered that the burn-off pulse alone will significantly reduce the background bias even with-out the use of the background correction algorithm.
In this experiment, ten sensors were stressed under 30°C and 91% humidity for 3 hours. Aqueous glucose at 50 mg/dL was used as sample. Five stressed sensors were tested with a 10 second burn-off pulse and five without the pulse. In addition, ten unstressed sensors were tested as control (five with the 10 second burn-off and five without) and the bias calculated using the following equation (3):
_ bias = ~"'~'' t""~°' x 10096 ( 3 ) i,=",.., It was found that the bias was 30.6% without the burn-off pulse and 18.0% with it which data demonstrate that the burn-off pulse alone reduces the background bias by about 40%.
MSE #1899 21706b0 Example III
This example explains how the algorithm corrects for background bias:
Eight sensors were stored at below -20°C for two weeks and another eight sensors were stressed at 50°C for four weeks. All sixteen sensors were tested using whole blood having a 100 mg/dL glucose concentration. The pa-rameter values were determined from fresh sensors. The glucose readings, G, were calculated as follows:
A. No background bias correction algorithm: Equa-tion 1 with k = 0.
B. Partial correction: Equation 1 with k = 0.8.
The bias in percent is calculated using Equation 4 with the results being listed in Table 1.
bias = G-100 X 100 (4) 2 5 a tas at 100 m~ldL
no burn-off partial cornection al orithm -__ 0.8 -20 C, 2 weeks 3.8 % 5.3 ~
50 C, 4 weeks 64.7 % 15.0 %
Tabl 1 B' MSE #1899 217~~560 A device capable of carrying out the invention is represented by FIG. 2. Referring to FIG. 2, there is shown a block diagram representation of a device for ac-s curately determining analyte values designated as a whole by the reference character 10 and arranged in accordance with principles of the present invention. Device 10 in-cludes a microprocessor 12 together with a memory device 14. Microprocessor 12 is suitably programmed to perform 10 the method of the invention as illustrated in FIG. 3.
Various commercially available devices, such as a DS5000 microcontroller manufactured by Dallas Semiconductor, can be used for the microprocessor 12 and memory 14. Memory 14 can be included within the microprocessor 12 or sepa 15 rately provided as illustrated in FIG. 2.
Digital data from the microprocessor 12 is applied to a digital-to-analog (D/A) converter 16. D/A converter 16 converts the digital data to an analog signal. An am-20 plifier 18 coupled to the D/A converter 16 amplifies the analog signal. The amplified analog signal output of am-plifier 18 is applied to a sensor 20.
Sensor 20 is coupled to an amplifier 22. The ampli-fied sensed signal is applied to an analog-to-digital (A/D) converter 24 that converts the amplified, analog sensor signal to a digital signal. The digital signal is applied to the microprocessor 12.
Various commercially available devices can be used for D/A converter 16, amplifiers 18 and 20 and A/D con-verter 24. For example, a device type PM-752F4FS manu-factured by PMITcan be used for D/A converter 16. Opera tional amplifier device type TL074AC manufactured and sold by Linear Technology can be used for amplifiers 18 and 22. A device type MAX 135 CWI manufactured and sold 5 by MaxumTM can be used~for the A/D converter 24.
Referring also to FIG. 3, there are shown the se-quential steps for accurate analyte determination of the invention. Initially microprocessor 12 applies a burnoff 10 pulse, for example a potential of 0.4 volts, to the sen-sor 20 as indicated at a block 300. Then the microproc-essor checks to identify a sample corresponding to a de-tected sensor threshold current value as indicated at a decision block 302. When a sample is detected at block 15 302, a predetermined burnoff time interval, such as 10 seconds is identified at a decision block 304. Next the current i1 is measured as indicated at a block 306 and an open circuit is applied to the sensor 20 as indicated at a block 308. Then a set delay or predetermined wait time 20 interval, such as fifteen (15) seconds is identified at a decision block 310. After the set delay, a read pulse or potential of 0.4 volts is applied to the sensor 20 as in-dicated at a block 312. Then a predetermined read time interval for the read pulse, such as 5 seconds is identi-25 fied at a decision block 314 and the current i2 is meas-ured as indicated at a block 316. Next microprocessor 12 gets the stored parameters for a particular sensor 20 in-cluding Int, slope, i1 lo, iz lo, S1 and k, as indicated at a block 320. The correction term Delta (i1, i2) is calcu-30 lated utilizing the stored parameters and measured burn-off current i1 and read current i2 as indicated block 322.
Next the analyte value, such as glucose reading G, is 2i7~6bU
calculated utilizing the read current i2 and the calcu-lated correction term Delta (i1, i2) multiplied by the se-lected scaling value k, as indicated at a block 324.
MSE #1899
Claims (11)
1. In a method for determining the concentration of an analyte in a fluid test sample by applying the fluid test sample to the surface of a sensor's working electrode which is electrochemically connected to a reference electrode which surface bears a composition comprising an enzyme specific for the analyte, a mediator which is reduced in response to a reaction between the analyte and the enzyme, which mediator has undergone partial reduction and then determining the concentration of the analyte in the fluid test sample as a function of a current which passes through the fluid test sample by measuring the current between the working and reference electrodes which results from the amount of reduced mediator produced during storage of the sensor at ambient conditions and during the time period, prior to a current measurement period and which is measured at the working electrode by applying a sufficient potential between the working and reference electrode to oxidize the reduced mediator at a time interval on the order of seconds after applying the potential, the improvement which comprises applying an oxidizing potential between the electrodes to return at least a portion of the mediator back to its oxidized form and switching to an open circuit or to a potential that substantially reduces the current to minimize the rate of electrochemical reaction at the working electrode for a set delay period before determin-ing the concentration of the analyte by applying a second potential between the electrodes and then measuring the current generated in the fluid test sample to thereby in-crease the accuracy of the analyte determination.
2. The method of claim 1 wherein the accuracy of the analyte determination is further increased by:
a) determining a first current i1 during the application of a positive pulse and a second current i2 at the end of the application of the second potential, and b) calculating the corrected analyte level G by solving the equation:
where Int and slope are the intercept and slope of i2, .DELTA.
(i.1, i2) is an error correction term proportional to a background bias calculated as :
where s1 = slope of i1.
i1-10 = i1 at a low analyte level, and i2-10 = i2 at a low analyte level.
a) determining a first current i1 during the application of a positive pulse and a second current i2 at the end of the application of the second potential, and b) calculating the corrected analyte level G by solving the equation:
where Int and slope are the intercept and slope of i2, .DELTA.
(i.1, i2) is an error correction term proportional to a background bias calculated as :
where s1 = slope of i1.
i1-10 = i1 at a low analyte level, and i2-10 = i2 at a low analyte level.
3. The method of claim 1 wherein the mediator is a ferricyanide salt.
4. The method of claim 2 wherein the analyte is glucose.
5. The method of claim 1 further includes the step of providing a selected time delay after applying said oxidizing potential before said second potential is applied.
6. The method of claim 1 wherein the step of applying said oxidizing potential between the electrodes includes the steps of applying a voltage potential selectively provided in a range between 0.1 volts and 0.9 volts, comparing the measured current with a threshold value and identifying a predetermined time interval for applying said voltage potential responsive to the measured current greater than said threshold value.
7. The method of claim 6 wherein said step of identifying a predetermined time interval includes identifying a set time interval selectively provided in a range between 5 seconds and 15 seconds.
8. The method of claim 5 wherein said step of providing a selected time delay includes the step of waiting a selected time delay in a range between 10 seconds and 40 seconds before said second potential is applied and wherein said second potential is applied for a selected time period in a range between 5 seconds and seconds before the step of measuring said current.
9. In a method for determining the concentration of an analyte in a fluid test sample by applying the fluid test sample to the surface of a working electrode which sur-face bears a composition comprising an enzyme specific for the analyte, a mediator which is reduced in response to a reaction between the analyte and the enzyme, which mediator has undergone partial reduction due to ambient conditions, the improvement which comprises:
a) applying a positive potential pulse to the electrode to oxidize at least a portion of the me-diator back to its oxidized form, b) determining a current (i1) during the applica-tion of the positive pulse and a current (i2) at the end of the application of a second potential, and c) calculating the corrected analyte level G by solving the equation:
where Int and slope are the intercept and slope of i2, .DELTA.
(i1, i2) is an error correction term proportional to a background bias calculated as:
where s1 = slope of i1 i1-10 = i1 at a low analyte level, and i2-10 = i2 at a low analyte level.
a) applying a positive potential pulse to the electrode to oxidize at least a portion of the me-diator back to its oxidized form, b) determining a current (i1) during the applica-tion of the positive pulse and a current (i2) at the end of the application of a second potential, and c) calculating the corrected analyte level G by solving the equation:
where Int and slope are the intercept and slope of i2, .DELTA.
(i1, i2) is an error correction term proportional to a background bias calculated as:
where s1 = slope of i1 i1-10 = i1 at a low analyte level, and i2-10 = i2 at a low analyte level.
10. The method of Claim 9 wherein the mediator is a ferricyanide salt.
11. The method of Claim 10 wherein the analyte is glucose.
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- 1995-05-05 US US08/435,993 patent/US5620579A/en not_active Expired - Lifetime
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- 1996-05-01 JP JP11063196A patent/JP3413323B2/en not_active Expired - Lifetime
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- 1996-05-09 US US08/647,219 patent/US5653863A/en not_active Expired - Lifetime
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| EP0741186A2 (en) | 1996-11-06 |
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