PROPOSAL FOR STANDARDIZING DIRECT-READING BIOSENSORS FOR BLOOD-GLUCOSE

Direct-reading glucose biosensors sense molality (glucose per unit wat er mass) in the sample. With aqueous calibration, a direct-reading glu cose biosensor produces higher results in blood and plasma than method s measuring concentration, theoretically by the ratio of water concent rations in calibrator and sample, To confirm this, we measured glucose in 140 blood and 40 plasma samples with the direct-reading glucose se nsor in the Chiron Model 860 Blood Gas and Critical Analyte System and with our routine method (ESAT 6660; Eppendorf). The Chiron instrument is calibrated with a 10 mmol/L (180 mg/dL) glucose calibrator (mass c oncentration of water = 0.99 kg/L). Assuming normal water concentratio ns of 0.84 and 0.93 kg/L in blood and plasma, respectively, we multipl ied results from the Chiron sensor by 0.84/0.99 and 0.93/0.99 to obtai n concentrations in blood and plasma. This conversion resulted in agre ement of results with our routine method. An individual conversion bas ed on hematocrit in each whole-blood sample was less satisfactory. To avoid confusion over variously measured and reported glucose results a nd reference values, we suggest standardization and reporting of whole -blood glucose results as equivalent plasma concentrations. This propo sal may be conveniently achieved by using a commercially available ref erence material for glucose, NIST SRM 965.