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Function and accuracy of glucose sensors beyond their stated expiry date.

BACKGROUND
The sensor of the Continuous Glucose Monitoring System (CGMS, Medtronic Minimed, Northridge, CA) is labeled to expire 6 months following its production and to measure the glucose concentration in interstitial fluid up to 3 days after insertion. The purpose of this study was to demonstrate potential possibilities of sensors when used beyond their expiry date.


METHODS
Twenty sensors, each between 3 to 18 months after the expiry date, were assessed in a 7-day period after insertion. Twenty healthy volunteers 23.4 +/- 2.92 (mean +/- SD) years old were trained in handling the CGMS and the Hypoguard (Woodbridge, UK) Advance glucometer system to measure their capillary plasma glucose concentration 18 times a day. Sensor function was estimated according to the number of readings per day, the accuracy according to the mean absolute difference (MAD), and correlation coefficient (r) between glucometer and sensor resulting from paired values.


RESULTS
Uninterrupted sensor function was found in 117 of 140 sensor-days (83.6%). A reduction of readings in 23 sensor-days (16.4%) was caused by user error (5 sensor-days, 3.6%), connecting cable (7 sensor-days, 5%), sensor failure (8 sensor-days, 5.7%), or uncertain factors (3 sensor-days, 2.1%). MAD was always < 28%, and r = 0.79.


CONCLUSIONS
Neither the expiry date nor the 3-day period of use limits the reliable function of a CGMS sensor. Sensors were found to function as long as 18 months after the expiry date, mostly for at least 7 days. There were no serious local adverse reactions. Prolongation of shelflife label and insertion time appears to be reasonable. Further studies are in progress.