Question

Which among the following is not used in the diagnosis of insulinoma?

• A. Fasting glucose test
• B. Xylulose test
• C. C-peptide levels
• D. Insulin / glucose ratio

Hint:

Three options test insulin biology; one is a pentose metabolism test.

Answer 1

The Correct Option is B

Step 1: Insulinoma is an insulin-secreting tumour of the pancreatic beta cells that causes hyperinsulinaemic hypoglycaemia. Diagnosis rests on demonstrating inappropriately high endogenous insulin during a hypoglycaemic episode.
Step 2: The standard workup centres on a supervised 72-hour fast (fasting glucose test). When blood glucose falls, insulin remains inappropriately elevated, confirming autonomous secretion.
Step 3: C-peptide is co-secreted with endogenous insulin in equimolar amounts, so high C-peptide proves the insulin is endogenous (and excludes surreptitious insulin injection). It is a key confirmatory test.
Step 4: The insulin / glucose ratio (amended insulin-glucose ratio) is calculated during the fast; a ratio that stays high when glucose is low (typically insulin in mU/L > glucose in mg/dL relationship) supports insulinoma.
Step 5: The xylulose test has no role here. Xylulose is a sugar linked to pentosuria and carbohydrate metabolism studies, not to insulin secretion. Therefore the xylulose test is the option not used.
Conclusion: The answer is option 2 (Xylulose test).