A 24-year-old woman presents with hypoglycaemia and high plasma insulin. To distinguish exogenous insulin administration from an insulinoma, which laboratory finding is most useful?

• A Low plasma C-peptide level accompanying the high insulin ✓
• B High plasma C-peptide level accompanying the high insulin
• C Elevated serum proinsulin-to-insulin ratio
• D Presence of anti-insulin antibodies in all cases of insulinoma

Explanation

Beta cells secrete insulin and C-peptide in equimolar amounts from proinsulin cleavage. Exogenous pharmaceutical insulin contains no C-peptide, and its presence suppresses endogenous secretion, so C-peptide falls despite high measured insulin. An insulinoma autonomously secretes both peptides together, giving elevated C-peptide, which eliminates option B. Anti-insulin antibodies can develop with any insulin exposure and do not distinguish tumour from injection, ruling out D.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.