Calibration of a subcutaneous amperometric glucose sensor. Part 1. Effect of measurement uncertainties on the determination of sensor sensitivity and background current.

The calibration of a continuous glucose monitoring system, i.e. the transformation of the signal I(t) generated by the glucose sensor at time (t) into an estimation of glucose concentration G(t), represents a key issue. The two-point calibration procedure consists of the determination of a sensor sensitivity S and of a background current I(o) by plotting two values of the sensor signal versus the concomitant blood glucose concentrations. The estimation of G(t) is subsequently given by G(t) = (I(t)-I(o))/S. A glucose sensor was implanted in the subcutaneous tissue of nine type 1 diabetic patients during 3 (n = 2) and 7 days (n = 7). For each individual trial, S and I(o) were determined by taking into account the values of two sets of sensor output and blood glucose concentration distant by at least 1 h, the procedure being repeated for each consecutive set of values. S and I(o) were found to be negatively correlated, the value of I(o) being sometimes negative. Theoretical analysis demonstrates that this phenomenon can be explained by the effect of measurement uncertainties on the determination of capillary glucose concentration and of sensor output.