MON-616 - Continuous Glucose Monitoring as an Adjunct Tool during A 72-Hour Fast in A Patient Evaluated for Insulinoma

Background:
The supervised 72-hour fast is the gold standard procedure in the evaluation of hypoglycemia when an insulinoma is suspected. Glucose levels are assessed intermitently using capillary or venous measurements, which may fail to capture dynamic glycemic fluctuations and are burdensome to patients. Hence, we suggest that the addition of continuous glucose monitoring (CGM) can enhance diagnostic precision, safety, and patient comfort, improving testing outcomes.

Case:
A 52-year-old woman with hypothyroidism and rheumatoid arthritis presented to our endocrinology clinic for the evaluation of episodic tremors, irritability, and diaphoresis occurring after meals, with home capillary blood glucose (BG) readings in the 70–80 mg/dL range. A 5-hour oral glucose tolerance test resulted in hyperinsulinemia followed by symptomatic biochemical hypoglycemia, suggesting post-prandial hypoglycemia. Metformin and acarbose were trialed, but discontinued due to gastrointestinal intolerance. A CGM (Dexcom G7) was then initiated. To exclude insulinoma, she was subsequently admitted for a supervised 72-hour fast while continuing CGM.

Over the course of the fast, venous BG levels declined gradually with appropriate physiologic ketosis, as her β-hydroxybutyrate increased from 0.15 to 3.47 mmol/L (normal 0.02–0.27). At approximately 48 hours, the patient developed neuroglycopenic symptoms accompanied by a capillary BG of 49 mg/dL and CGM reading of 64 mg/dL. STAT laboratory evaluation revealed a venous BG of 56 mg/dL with a β-hydroxybutyrate of 3.47 mmol/L. Given symptomatic hypoglycemia and marked ketosis suggestive of absent insulin production, the fast was terminated. A 1-mg glucagon challenge was performed, with minimal glycemic response (fingerstick BG 52→57 mg/dL; CGM 65→70 mg/dL at 30 minutes). Insulin and C-peptide levels remained appropriately suppressed throughout the fast, ranging between < 1.0–3.6 µU/mL (normal < 18.4) for insulin and 0.23–0.6 ng/mL (normal 0.80–3.85) for C-peptide. This excluded insulinoma and supported a diagnosis of fasting hypoglycemia due to starvation physiology. CGM values modestly overestimated BG during hypoglycemia but closely followed capillary trends, allowing near real-time clinical correlation and enhanced safety monitoring.

Conclusion:
This case illustrates the practical use of CGM as an adjunct during the 72-hour fast, providing continuous glycemic context and supporting real-time clinical decision-making. The CGM data paralleled capillary glucose values, offering confirmation of glucose stability and early detection of hypoglycemia for both the patient and her care team. Although CGM modestly overestimated glucose at lower ranges, the continuous data stream reduced the need for frequent fingerstick testing and improved patient comfort. Future studies should evaluate the diagnostic accuracy of CGM in this setting.

*Unless otherwise noted, all abstracts presented at ENDO must not be released to the press or the public until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.*