Calibration of a subcutaneous amperometric glucose sensor implanted for 7 days in diabetic patients - Part 2. Superiority of the one-point calibration method

Calibration, i.e. the transformation in real time 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 for the development of a continuous glucose monitoring system. Objective: To compare two calibration procedures. In the one-point calibration, which assumes that 10 is negligible, S is simply determined as the ratio I/G, and G(t) = I(t)/S. The two-point calibration consists in the determination of a sensor sensitivity S and of a background current I,, by plotting two values of the sensor signal versus the concomitant blood glucose concentrations. The subsequent estimation of G(t) is given by G(t) = (I(t)-I-o)/S. Research design and methods: A glucose sensor was implanted in the abdominal subcutaneous tissue of nine type I diabetic patients during 3 (n = 2) and 7 days (n = 7). The one-point calibration was performed a posteriori either once per day before breakfast, or twice per day before breakfast and dinner, or three times per day before each meal. The two-point calibration was performed each morning during breakfast. Results: The percentages of points present in zones A and B of the Clarke Error Grid were significantly higher when the system was calibrated using the one-point calibration. Use of two one-point calibrations per day before meals was virtually as accurate as three one-point calibrations. Conclusion: This study demonstrates the feasibility of a simple method for calibrating a continuous glucose monitoring system. (C) 2002 Published by Elsevier Science B.V.