The insulin-to-C-peptide ratio can be used to distinguish factitious insulin-induced hypoglycemia from insulinoma.
The molar ratio of insulin to C-peptide is low (typically < 0.2 according to Tietz) in the peripheral circulation due to the longer half-life of C-peptide in the circulation (half-life C-peptide 20 minutes, half-life insulin 5-10 minutes). Insulin is degraded by the liver and has an increased half-life in cirrhosis. C-peptide is degraded by the kidney and has an increased half-life in renal failure.
insulin-to-C-peptide ratio =
= (insulin in nmol/L) / (C peptide in nmol/L)
Interpretation:
• A ratio < 1 indicate increased endogenous insulin secretion (insulinoma, sulfonylurea administration) or renal failure.
• A ratio > 1 indicates exogenous insulin administration or cirrhosis.
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