A USER-FRIENDLY METHOD FOR CALIBRATING A SUBCUTANEOUS GLUCOSE SENSOR-BASED HYPOGLYCEMIC ALARM

A crucial step in developing a glucose monitoring system using a subcu taneous implanted glucose sensor is the transformation of the sensor s ignal (a current) into an estimation of a blood glucose concentration. We have developed an Electronic Control Unit (ECU)able to recognize, before and after a glucose load, that the sensor current presents a pl ateau, thus triggering an alarm asking for blood glucose determination . The system, fed with these results, subsequently transforms the curr ent into an estimation of glucose concentration by linear extrapolatio n based on the sensor sensitivity and the background current computed from the two sets of current and glycaemia values (two-point calibrati on). In addition, the system is able to trigger an alarm when this est imation decreases below a threshold that can be set by the user. This system was evaluated in experiments performed in 12 normal rats. The q uality of the calibration was assessed by comparing, by error grid ana lysis, the data displayed on the liquid-crystal display of the ECU to concomitant plasma glucose concentration determined at frequent interv als. 65 +/- 6 and 26 +/- 5% of the values were in zones A (good) and B (acceptable estimation) of the grid, respectively. The system was set to trigger an alarm when the estimation of glucose concentration decr eased below 70 mg/dl. Following an insulin administration, the alarm w as triggered when the system displayed a 64 +/- 2 mg/dl glucose concen tration. The concomitant plasma glucose concentration was 59 +/- 5 mg/ dl (NS). In conclusion, this work validates experimentally the new, us er-friendly method for calibrating the glucose sensor integrated into the ECU, based on an automatic detection of plateaus. The quality of t he sensor calibration performed with this procedure is compatible with the appropriate functioning of this continuous glucose monitoring sys tem, which was demonstrated by its ability to detect mild hypoglycaemi a following insulin injection. (C) 1997 Elsevier Science Limited.