C RITICAL C ARE. Davide Colombo Kevin Flanagan Kevin Fallon. The IL Glucose Biosensor: Interfering Substance Studies.

Size: px

Start display at page:

Download "C RITICAL C ARE. Davide Colombo Kevin Flanagan Kevin Fallon. The IL Glucose Biosensor: Interfering Substance Studies."

Herbert Glenn
5 years ago
Views:

1 C RITICAL C ARE 5 Davide Colombo Kevin Flanagan Kevin Fallon The IL Glucose Biosensor: Interfering Substance Studies.

2 Davide Colombo - Instrumentation Laboratory Kevin Flanagan - Instrumentation Laboratory Kevin Fallon - Instrumentation Laboratory The IL Glucose Biosensor: Interfering Substance Studies.

3 Introduction 3 The technology utilized for determining glucose in whole blood is based on an enzymatic system. This technology requires membranes that have consistent formulation. The membranes must also form a barrier to common substances that might interfere with the measurement of glucose. An earlier monograph, Glucose Biosensor for Whole Blood Analysis described the technology of the IL glucose sensor. This paper describes the interference studies carried out on the IL glucose sensor.

4 4 The IL Glucose Sensor Technology As stated in the previous page, the glucose sensor has two components: an outer membrane system and an inner electrochemical sensor (see picture below): The outer membrane system has three components: 1. The outer polycarbonate membrane that allows glucose to diffuse though to the middle layer but which bars the diffusion of other larger molecules such as plasma proteins. 2. The middle layer containing enzymes that convert glucose to H 2 O 2 (Hydrogen Peroxide). 3. The inner cellulose acetate membrane that allows the H 2 O 2 to diffuse through to the sensor. The composition of this inner membrane is critical since it must prevent the diffusion of other substances that might react with the sensor and affect the glucose measurement. The sensor is maintained with a polarizing voltage equal to 750 mv. The oxidation of the H 2 O 2 formed above at the positive pole of the electrode creates a net flow of electrons, which is measured as an electrical current. Therefore, the formation of H 2 O 2 and its detection by the sensor is directly proportional to the concentration of glucose and is linear to very high concentrations of glucose (up to 500 mg/dl/27.7 mmol/l). As stated above, the inner or cellulose acetate membrane is critical for separating the sensor from any other materials that might react at this polarizing voltage. 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: IL Glucose sensor The sensor is calibrated automatically with the CAL1 and CAL 2 solutions These are the same solutions utilized to calibrate ph, sodium, potassium, ionized calcium and chloride on the Synthesis system. The concentration of glucose is 0 mg/dl in the CAL 1 and 90 mg/dl (5 mmol/l) in the CAL 2. The complete calibration of the channel is carried out automatically by the analyzer at a frequency selected by the user. In addition, the analyzer performs automatically a CAL 2 glucose calibration every hour. The CAL 2 glucose calibration cycle requires 2 minutes from start to finish. This cycle allows the membrane to be periodically calibrated on the high glucose level.

5 Materials and Methods 5 Table 1 lists the substances and concentrations that were tested. The concentration value is the final concentration in plasma samples. Interferent 4-amino.acetophenol Urea Creatinine O-Acetyl salicic acid Ammonium thyocianate Ascorbic Acid Concentration 1,30 mmol/l 5,30 mmol/l 0,13 mmol/l 3,00 mmol/l 2,40 mmol/l 0,20 mmol/l Table 1: Interfernce substances tested and their concentration Samples: Plasma samples were divided into two aliquots. One aliquot was untreated (as drawn) and the other was treated to bring the concentration of interfering substance to the desired level (table 1). Systems: Four IL Synthesis 35 systems were used for analysis. Two systems had newly installed glucose caps - to reproduce the conditions of glucose membrane at installation - and two had caps that had been installed for more than 8 hours. Table 2 presents the actual values for each system. Analysis: Five replicates of each sample were analyzed on each of the four systems. The average value for each condition was referenced to YSI 2300 analyzer (our comparison system) and the bias calculated.

6 6 Results The following table shows the electrochemical values of the four glucose membranes at time=0 Analyzer # Offset Slope Table 2: Offset and slope values of the glucose electrodes The following table summarizes the results obtained on plasma samples containing the different interferent substances: Instr. # No Int. Int.1 No Int. Int.2 No Int. Int.3 No Int. Int.4 No Int. Int.5 No Int. Int.6 No Int. Int ,00 69,00 70,00 71,00 71,00 69,00 71,00 69,00 62,00 64,00 61,00 61,00 62,00 62,00-1,00 1,00-2,00-2,00 2,00 0,00 0, ,00 75,00 73,00 72,00 72,00 72,00 72,00 71,00 64,00 63,00 61,00 62,00 61,00 62,00 1,00-1,00 0,00-1,00-1,00 1,00 1, ,00 74,00 73,00 69,00 72,00 71,00 71,00 70,00 63,00 64,00 61,00 59,00-1,00-4,00-1,00-1,00 1,00-2, ,00 71,00 69,00 69,00 69,00 67,00 68,00 66,00 60,00 59,00 59,00 59,00 59,00 58,00 1,00 0,00-2,00-2,00-1,00 0,00-1,00 Table 3: Effect of various interferents on IL glucose sensor (all data in mg/dl) The even columns (identified as No Int. ) report the values obtained by analyzing samples without any interferent. The odd columns report the number of interferent added to the sample, where: Int. 1= 4-amino acetophenol Int. 2= Urea Int. 3=Creatinine Int.4=O-acetyl salicylic acid Int. 5= Ammonium Thyocianate Int. 6=Ammonium chloride Int.7= Ascorbic Acid Below each result obtained on the samples containing the interfering substance, we have also reported the mean bias obtained with respect to the reference value.

7 Discussion and Conclusions 7 The objective of this study was to test the effect of the most common interfering substances in plasma on the glucose measurement of the IL Synthesis blood gas system. The results obtained show the following average bias on all plasma samples: Interferent Average Bias 4-amino-acetophenol 0 mg/dl (0 mmol/l) Urea -1,00 mg/dl (-0,06 mmol/l) Creatinine -1,25 mg/dl (-0,07 mmol/l) O-acetyl salicilic acid -1,75 mg/dl (-0,09 mmol/l) Ammonium thiocianate 0,25 mg/dl (0,01 mmol/l) Ammonium Chloride 0,33 mg/dl (0,02 mmol/l) Ascorbic Acid -0,50 mg/dl (0,03 mmol/l) Table 4: Average bias on glucose caused by the presence of interferences. Glucose levels tested in the range of mg/dl (3,3-4,1 mmol/l) All of the above biases are considered negligible from the clinical point of view. Also, the results were obtained on membranes that showed different behavior at installation. Based upon the above results and consideration, it may be concluded that the IL glucose membrane is not affected by the above mentioned interferents. Bibliography: Ideal Hydrogen peroxide-based glucose sensor Palleschi et al Applied Biochemistry and Technology vol 31 p Biosensor for blood glucose: a new question of what is measured and what should be reported Thomas C. Maley (Ph.D. Ciba Corning Diagnostic S.A.)and Paul D Orazio (PhD Instrumentation Laboratory) Glucose Biosensor for whole blood analysis Manzoni A, Zanardi S, Zonca P, Johnson J IL Critical Care Monograph # 2 Yellow Spring Instruments, Instruction Manuals, Model 2300 Glucose analyzer, Yellow Spring Instrument Company Inc., Yellow Spring OHIO IL Synthesis Operator s Manual - Instrumentation Laboratory Lexington Mass.

8 1999 Instrumentation Laboratory - Printed in Italy - Grafica Briantea

9 Part. No

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

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