Factory-calibrated continuous glucose monitoring and capillary blood glucose monitoring in a case with insulinoma: usefulness and possible pitfall under chronic hyperinsulinemic hypoglycemia

The accuracy of factory-calibrated continuous glucose monitoring (fCGM) within hypoglycemic ranges, especially under the status of chronic hyperinsulinemic hypoglycemia like insulinomas, remains an issue. Even so, fCGM is known to be useful for detecting hypoglycemia unawareness in insulinoma cases. A 25-year-old woman presenting with sudden unconsciousness was diagnosed with insulinoma; ICGM facilitated diagnosis by continuous monitoring for hypoglycemia. Before surgery, she was treated with continuous and frequent bolus infusions of 50% glucose via central venous catheter. To evaluate the accuracy of fCGM values in this case, a comparison between fCGM and capillary blood glucose (CBG) values was also performed. According to the simultaneously measured values, those of ICGM were largely in accordance with those of CBG. Moreover, compared with the previously reported case not having glucose infusions via central venous catheter, both the mean absolute relative differences (MARDs) and the absolute differences (Delta glucose) between MGM and CBG values were larger in the present case, although no significant differences of MARDs and Delta glucose between the two cases were observed in several different conditions including fasting, post-meal, hypoglycemia, and others. Therefore, we should note possible increased differences between fCGM and CBG values in cases using frequent intravenous glucose infusions as well as case-dependent differing levels of consistency between them.