Renee Scott, Brad S. Karon, Laurie Griesmann, Sandra C. Bryant, Jeffrey A. DuBois, Terry L. Shirey, Steven Presti, and Paula J.

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1 ABSTRACT We evaluated four hospital-based glucose meter technologies for accuracy, precision, and analytical interferences likely to be encountered in critically ill patients. Precision of all four glucose meters, both within run and day-to-day, instruments. Accuracy (bias) of the meters and analytical interference were evaluated by comparing results obtained on whole blood specimens to plasma samples obtained from these whole blood specimens run on a hexokinase reference method. There were significant differences in the degree to which the meters correlated with a reference hexokinase method. Ascorbic acid showed significant interference with 3 of the 4 meters. Hematocrit also affected the correlation between whole blood and plasma hexokinase glucose on 3 of the 4 glucose meters tested, with the magnitude of this interference also varied by glucose meter technology. The use of glucose meters to maintain tight glycemic control in critically ill patients may be impacted by the accuracy and hematocrit dependence of some glucose meter technologies. GLUCOSE METHODS USED Accu-Chek Inform (Roche Diagnostics, Indianapolis, IN) Glucose dehydrogenase based amperometric strip Precision PCx (Abbott Diabetes, Alameda, CA) Glucose oxidase amperometric detection SureStepFlexx (LifeScan, Malpitas, CA) Photometric glucose oxidase detection StatStrip (Nova Biomedical, Waltham, MA) Modified glucose oxidase based amperometric test system Roche Integra 400 Analyzer (Roche Diagnostics, Indianapolis, IN) Hexokinase method TABLE 1 Within Run and Day-to-Day precision for glucose meters, in percent coefficient of variation (CV), n= number of replicates tested Within Run Precision, Day-to-Day Precision, CV (%) n = 20 CV (%) n = 30 Meter Low Medium High Low High Technology Glucose Glucose Glucose Glucose Glucose mg/dl mg/dl mg/dl mg/dl mg/dl StatStrip Accu-Chek PCx SureStep The coefficient of variation (CV) obtained from both within-run and day-to-day precision experiments was less than 5% for all meters tested except day-to-day precision at low glucose on PCx which was 5.1% STUDY DESIGN Method correlation was performed by analyzing 133 whole blood specimens and 52 spiked whole blood patient specimens on the 4 glucose meters, compared to plasma obtained from those specimens and run on the Integra (reference method). Percent bias (meter result minus plasma result/plasma result) plotted as a function of hematocrit for the 133 whole blood specimens. Interference studies were performed using ascorbic acid added to whole blood at 4 different glucose levels. Six strips tested on each meter at each concentration of interferant, mean used for data analysis. Clinically significant interference defined as > 10 mg/dl(for glucose concentration < 100 mg/dl) or > 10% (for glucose concentration > 100 mg/dl) change from baseline. Hematocrit interference was tested using 3 glucose concentrations over a 25-60% hematocrit range, using six strips for each meter at each hematocrit range tested. Clinically significant effect of hematocrit defined as > 10 mg/dl(for glucose concentrations < 100 mg/dl) or > 10% (for glucose concentrations > 100 mg/dl) change between lowest and highest hematocrit tested. TABLE 2 Correlation data for glucose meters vs reference plasma hexokinase method, n=185 or 133 Meter Slope Intercept (mg/dl) r 2 Median bias (mg/dl) Technology N=185 N=133 N=185 N=133 N=185 N=133 N=185 N=133 StatStrip Accu-Chek PCx SureStep n=133 data set, whole blood samples n=185 data set, 133 whole blood samples and 52 spiked whole blood samples StatStrip and Accu-Chek meters correlate better with the reference hexokinase method, as measured by slope and intercept Median bias was lower for the StatStrip and SureStep meters 2007 Mayo Foundation for Medical Education and Research 11 1

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4 Glucose meter percent bias was plotted against hematocrit values for the 133 patient sample set (no sample manipulation). There is a clear trend for negative bias associated with increasing hematocrit for the PCx and SureStep meters. The Accu-Chek and StatStrip were largely unaffected by hematocrit. 14

5 CONCLUSION Glucose meters vary in the degree of correlation to the hexokinase reference glucose methods. Most glucose meters continue to be affected by analytical interferences, especially hematocrit. The new StatStrip glucose meter technology correlated closely with a reference hexokinase glucose method and did not demonstrate clinically significant interference from any of the substances tested. 15