C RITICAL C ARE 2. Angelo Manzoni, Susanna Zanardi, Pietro Zonca Jay Johnson. Glucose biosensor for whole blood analysis

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1 C RITICAL C ARE 2 Angelo Manzoni, Susanna Zanardi, Pietro Zonca Jay Johnson Glucose biosensor for whole blood analysis

2 *Angelo Manzoni, *Susanna Zanardi, *Pietro Zonca **Jay Johnson * Instrumentation Laboratory SpA - Viale dell Industria, Paderno Dugnano (Milano) ** Yellow Springs Instrument Co., Inc. - P.O. Box Yellow Springs, OHIO (USA) Glucose biosensor for whole blood analysis

3 ABSTRACT The determination of glucose in whole blood on blood gas and electrolytes analytical systems has been accomplished through the coupling of an Hydrogen Peroxide electrochemical sensor with an enzymatic system. Glucose in the sample diffuses to the enzymatic layer and reacts producing Hydrogen Peroxide, an electroactive molecule which is detected by the amperometric system. The H 2 O 2 sensor uses platinum for the anode (working electrode) and Ag/AgCl for the cathode (reference electrode) The applied voltage between the working and the reference electrode is 75 mv because hydrogen peroxide oxidation and silver chloride reduction occur at this applied potential and the measured current intensity is linear with the production of hydrogen peroxide and the glucose concentration in the sample. The applied potential can support the oxidation of other interfering substances present in the sample. An electrode configuration has been combined with a specific membrane that minimizes both the effects of the interfering substances and of Oxygen of the sample. INTRODUCTION This work describes the development of a whole blood glucose biosensor and its integration into an existing fluidic module where other analytes, including blood gases, are measured. Description of the analytical module: The glucose sensor is compatible with whole blood, serum and plasma. Analytical sensors: ph, pco 2, po 2, sodium,potassium, Hct% and glucose. Sample volume: 24 µl Aspiration time: 8 sec. at 3 µl/sec The calibration curve is obtained with two calibrating solutions: Cal 1: no glucose, HEPES/NaHEPES buffer 5 mm ph at 37 C Cal 2: 5mM glucose, MOPS/NaMOPS buffer 5 mm ph 6.84 at 37 C Analysis time: 9 sec. Maximum Linearity range: 1-5 mg/dl Comparison tecnique: GOD/POD procedure IL Monarch Analyzer Total allowable error: < 1% Maintenance procedure frequency: Enzymatic cap replacement after approximately one month use life 3

4 METHODOLOGY Figure 1 presents the enzymatic membrane.(1) The pore size and pore density of the outer policarbonate membrane allows glucose and oxygen within the sample to diffuse through this membrane in order to react at the enzyme layer where H 2 O 2 is produced.(2) The Hydrogen Peroxide produced by the above reaction diffuses through the inner cellulose acetate membrane and reacts at the surface of the platinum electrode. When the applied potential is 75 mv (relative to the reference electrode) the current intensity is linearly related to the glucose concentration in the sample. Other molecules can diffuse and react at the platinum electrode surface and this may reduce the selectivity of the sensor. EXPERIMENTAL RESULT Sensor assembly Figure 2 shows the configuration of the sensor including the enzymatic glucose cap, which is the disposable part of the sensor containing the glucose selective membrane, and the electrode. Glucose diffuses from the sample through the membrane providing electrochemical connection between the platinum working electrode and the Ag/AgCl (reference electrode) surface. Fluidic diagram Figure 3 depicts the layout of the fluidic module of the Synthesis 35 and the positioning of the glucose electrode relative to the other electrodes. Response Figure 4a shows the typical response of the sensor during the typical calibration cycle. The actual measurement is determined when the variation of the signal is within +/-.5 % of the actual signal on 9 consecutive data points taken within 5 seconds. The time required to satisfy this constraint varies from 6 to 9 seconds. Figure 4b demonstrates that the calibration curve is obtained between the baseline current (zero glucose) and 9 mg/dl of glucose. After each sample the recovery of the baseline is continuously monitored. Installation Figure 5 shows the changes in the electrode sensitivity immediately after the installation in the analytical module. The enzymatic membrane adjusts its sensitivity taking into account the temperature (37 C) and the wetting condition. In order to compensate for the changes in membrane sensitivity without compromising analytical performance immediately after the installation, the frequency of 2 points calibration has been fixed every 15 minutes for the first two hours. Sensitivity Figure 6 illustrates the distribution of the sensitivity of different glucose enzymatic caps (about 15 caps). Most of the caps show similar sensitivity. 4

5 Recovery The schematic in figure 7 shows the procedure to asses the linearity of the system: 5 ml of whole blood was separated into two aliquots. In one aliquot the glucose concentration was increased substantially. These two aliquots were then mixed in varying proportions to give varying glucose concentrations without changing the hematocrit. The figure 8 shows the result obtained. Hematocrit effect In figure 9 the procedure used to study the hematocrit effect has been described. Different glucose levels from 6 to 4 mg/dl at different hematocrit (2-7%) were thus prepared. HCT results Figure 1 indicates the hematocrit effect on electrode response at the different glucose concentrations. The solid line represents the plasma real values given by the electrode. The square symbols indicate changes in electrode response due to changes in hematocrit variations from 2% to 7% at different glucose concentration. The triangle symbols represent the response of the electrodes when hematocrit corrections are applied at the different glucose concentrations. The hematocrit correction algorithm was obtained through experimental procedures. The glucose concentration in blood was plotted against the glucose concentration in plasma as measured by the same electrode. Different slopes (m) for different hematocrit levels were determined. The different slopes were plotted against the Hct values and the correction algorithm determined (see next page). Figure 11 shows the results of a study comparing these sensors with the YSI 23 Glucose Analyzer. Figure 12 shows the comparison to an automatic clinical chemistry analyzer (IL Monarch - colorimetric technique). A single glucose cap showed the imprecision to be 3% on whole blood and 2% on plasma samples over a period of 2 days at 7 mg/dl of glucose. Aqueous control solutions showed imprecision that was less than 2% from 6 to 3 mg/dl over the same period. During this time no decrease of sensitivity of the electrodes was noticed. Oxygen effect Figure 13 shows that po 2 levels between 5 and 18 mmhg had no effect on aqueous samples containing 3 mg/dl of glucose. 5

6 Hematocrit correction algorithm Hematocrit influence on electrode behaviour. m=1 Hct= Glucose concentration (blood) m m1 Hct=2 m2 Hct=4 m3 Hct=6 Glucose concentration (plasma) GLU (blood) = GLU (plasma) x m + b then b= Hct1=m1 Hct2=m2 Hct 1 Hematocrit influence on slope (m) slope (m) 1 Hematocrit % m=1-k(hct) GLU (blood) = GLU (plasma) x (1-K (Hct)) GLU (plasma) = GLU (blood) / (1-K (Hct)) 6

7 Interferents The effect.of uric acid, ascorbic acid and acetaminophen (interfering substances) has been studied in aqueous solution at a concentration of 9 mg/dl of glucose at two different glucose electrodes sensivities. (3) Both electrodes show a negligible effect with uric acid and ascorbic acid. Acetaminophen on the contrary at a level of 1 mg/dl gives about 1 mg/dl of glucose equivalent. Interferent normal value Max value Acetaminophen 4-5 mg/dl 15 mg/dl *(YSI 1.5 mg/dl=5mg/dl di gluc ) Ascorbic Acid >.5 mg/dl *(YSI 28 mg/dl=5mg/dl di gluc ) Uric Acid 12 mg/dl *(YSI 4 mg/dl=5mg/dl di gluc ) *( YSI value from YSI 23 Manual) Interferent conc Cal 2 Cal2+ diff. Cal 2 Cal2+ diff. mg/dl interf mg/dl interf mg/dl gluc gluc Acetaminophene Ascorbic Acid Uric Acid CONCLUSIONS The analytical performances and the use life obtained demonstrate that whole blood glucose could be successfully measured in blood gas and electrolytes analytical equipments (IL Synthesis 3/35 and IL 166) using a biosensor obtained coupling an electrochemical hydrogen peroxide sensor with an enzymatic system. The oxygen effect has been studied and no effect has been observed. The effect of interfering substances was negligible and the hematocrit effect has been corrected through the experimental algorithm described. REFERENCES (1) Yellow Spring Instruments, Instruction manuals, Model 23 Glucose analyzer, Yellow Spring Instruments Company Inc. Yellow Spring Ohio (2) Ideal hydrogen peroxide-based glucose sensor Palleschi et al - Applied Biochemistry and Biotecnology vol 31 p (3) Biosensor for blood glucose: a new question of what is measured and what should be reported Thomas C. Maley and Paul D Orazio PhD Ciba Corning Diagnostic S.A. 7

8 Electrode O2 Cellulose acetate Enzyme layer Polycarbonate membrane Reactions H O 2 2 Enzyme layer β-d-glucose + O 2 ---> Glucono-δ-lactone + H 2O 2 Cathode AgCl e > Ag + Cl 4H + O 2 ---> 2H 2O 2-4e Anode H O ---> 2H O 2+ 2e Glucose Interferents Oxygen Proteins/cell Figure 1 Membrane cap and electrode assembly Membrane Rubber Cap Cuvette Electrode gasket Silver/Silver-chloride counter & reference electrode insulating layer ø,5 mm Platinum anode (working) Figure 2 8

9 Figure 3 9

10 5 sec d 5 sec S d.5 S mg/dl Blood sample Typical Sensor Response Chart speed cm/m Calibration cycle cm CAL 2=9 g/dl Priming.8 na 2 mg/dl Blood sample CAL = mg/dl 1 cm/m Figure 4a - 4b 1

11 Electrode installation (slope change) pa/mm Slope pa/mm tim (m nutes) 9 4 Figure 5 11

12 Tipical slopes distribution Frequency Cumulative % value (pa/mm) Figure 6 12

13 Method of dilution to obtain blood samples with different glucose level 5 ml hole blood 6 mg/dl glucose Glucose A B 3 ml whole blood 6 mg/dl glucose 2 ml whole blood 45 mg/dl glucose 6 ml A + ml B 5 ml A + ml B 4 ml A +2 ml B 3 ml A +4 ml B ml A +5 ml B ml A +6 ml B Dilution ratio Figure 7 13

14 Linearity: Glucose recovery 6 Slope = 4.27 offset = R = glucose (mg/dl) replicates for each level Dilution ratio Figure 8 14

15 Method of dilution t obtain blood samples with different Hematocrit evel 5 ml whole blood 6 mg/dl glucose 4-45% Hct Glucose min centrifugation 5 ml whole blood X mg/dl glucose A B 2 ml plasma Hct%= 3 ml Blood Hct%=75 mix 6 ml A + ml B 5 ml A + ml B 4 ml A +2 ml B 3 ml A +4 ml B ml A +5 L B ml A +6 ml B Dilution ratio Figure 9 15

16 instruments 4 replicates each level Hematocrit effect: raw data, corrected data 2 7 Hct% mg/dl Glu plasma real value GLU raw data GLU calc sample Figure 1 16

17 Hct corrected data vs YSI 23 instrument 6 2 instruments, 4 replicates each level BG Gluc. electrode 2 Intercept= Slope= R =.947 Observation=418 1 Figure YS (mg/dl) 17

18 Hct corrected data vs IL Monarch instrument 6 2 instruments, 4 replicates each level 5 4 BG Gluc electrode 3 2 Intercept= Slope= R =.99 Observation= Monarch (mg/dl) Figure 12 18

19 Oxigen sensitivity of glucose electrode with 3 mg/dl acqueous solution pa 5 pa po (mmhg) 2 Figure 13 19

20 1998 Instrumentation Laboratory - Printed in Italy - Grafica Briantea - 298

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