Insulin C-peptide (connecting peptide), a 31 amino acid polypeptide, represents the midportion of proinsulin. During insulin secretion it is enzymatically cleaved from proinsulin and co-secreted in equimolar proportion with mature insulin. The half life of C-peptide is significantly longer than insulin, so it is detectable in higher concentrations and the level less variable. C-peptide is often a more reliable marker than insulin. In addition, insulin is destroyed by proteases in haemolysed samples, while C-peptide is not.

Clinical uses:

1. Insulinoma: elevated C-peptide levels from increased beta-cell activity

2. Covert self-administration of insulin: can be virtually ruled out as cause of hyperinsulinaemia by finding elevated C-peptide.

3. Type 1 diabetes mellitus: low C-peptide levels due to diminished insulin secretion, or suppressed as a normal response to exogenous insulin. Patients on insulin can develop anti-insulin antibodies which can interfere with insulin assay, so C-peptide can be used instead to check residual beta-cell activity.

Sample Requirements and Reference Ranges