What test confirms the diagnosis of insulinoma?

• A. 72-hour fasting measurement of insulin and glucose
• B. Random glucose measurement
• C. Oral glucose tolerance test
• D. HbA1c

Answer: (A)

The key idea is to demonstrate endogenous hyperinsulinemic hypoglycemia in a controlled setting. The best way to confirm an insulinoma is a supervised 72-hour fast that continues until hypoglycemia occurs (or up to 72 hours). During the fast, you measure glucose and insulin levels at the time of hypoglycemia. If insulin remains inappropriately high when glucose is low, that shows the pancreas is secreting insulin autonomously. Measuring C-peptide (and proinsulin) helps confirm the insulin is coming from the body's own pancreas rather than from external insulin, which is crucial because external insulin would have low C-peptide. This combination provides definitive biochemical evidence of endogenous hyperinsulinemic hypoglycemia due to an insulin-secreting tumor.

Random glucose measurements can miss the pattern and aren’t diagnostic for this condition, the oral glucose tolerance test isn’t the standard diagnostic tool for insulinoma, and HbA1c reflects long-term average blood sugar rather than the acute hypoglycemia seen with insulinoma.