Serum C-Peptide

Function

C-peptide is a byproduct created when the pancreas produces insulin. Proinsulin is cleaved into one molecule of insulin and one molecule of C-peptide. Because C-peptide is secreted in equimolar amounts to insulin but has a longer half-life and is not cleared by the liver as quickly, it provides a much more stable and accurate reflection of endogenous insulin production.

Why it is Ordered

This test is primarily used to distinguish between Type 1 and Type 2 Diabetes. In Type 1, the pancreas produces little to no insulin (and thus no C-peptide). In Type 2, the body is resistant to insulin, often resulting in normal or high C-peptide levels. It is also used to investigate the cause of hypoglycemia (low blood sugar)—for instance, to see if a patient is secreting too much of their own insulin (Insulinoma) or if they took too much exogenous insulin (where C-peptide would be low).

Associated Conditions

• Type 1 Diabetes: Characterized by low or undetectable C-peptide.
• Type 2 Diabetes/Insulin Resistance: Characterized by normal to high C-peptide.
• Insulinoma: A pancreatic tumor that secretes excessive insulin and C-peptide.
• Factitious Hypoglycemia: Use of external insulin causes low blood sugar but low C-peptide.