Continuous subcutaneous glucose monitoring: good enough to use in glucose regulation protocols?

Abstract

Tight glucose regulation is benefi cial in subgroups of inten sive care unit (ICU) patients, but may harm other sub groups. Th is harm may be due to hypoglycemic events. In avoiding hypoglycemia, an accurate bedside glucometry method is essential [1]. Bridges and colleagues therefore evaluated the accuracy of a continuous subcutaneous glucose monitoring system (CGMS) in critically ill children, and concluded that the system proves highly accurate [2]. We have a problem with this resolute conclusion. Th ey report a Pearson’s correlation coeffi cient of 0.68, which is quite low. Vlkova and colleagues even conclude that, based on a correlation coeffi cient of 0.69 comparing subcutaneous glucose values and laboratory blood glucose values in 15 patients, subcutaneous devices should not be used in critically ill patients [3]. We found a correlation coeffi cient of 0.87 in evaluating the same CGMS in 60 critically ill patients, but were concerned with the inaccuracy in the low glucose zone: we found a diff erence of nearly 4 mmol/l (reference blood glucose 2.8 mmol/l versus subcutaneous sensor 6.5 mmol/l) in one patient [4]. Bridges and colleagues report 142 subcutaneous glucose readings <2.2 mmol/l that were falsely low, checked against blood glucose values. Th e Clarke error grid is a better way to evaluate the accuracy of a CGMS than Pearson’s correlation coeffi cient. In most published studies, the deviation of subcuta neous measurements stays in the (wide) clinically accep table zones of the Clarke error grid. Th ese deviations of the CGMS system, however, when used in a tight glucose regulation protocol – and adjusting the insulin dose based on the subcutaneous readings – could have severe consequences in the individual patient, if the deviations result in an unjust rise in insulin dose. Since computerized protocols based on arterial blood samples give excellent glucose regulation with a negligible chance of hypoglycemic events [5], we decided to continue using this computerized protocol to avoid treatment-related morbidity. Subcutaneous CGMS seems not good enough in aiming for tight glucose regulation in the ICU. Intravascular CGMS, used in a closed feedback loop with insulin infusion, is promising, but has not yet been evaluated in clinical studies in critically ill patients.