C-peptide measurement (EFO_0005187) is a measurement of the connecting peptide, or C-peptide -  a short 31-amino-acid polypeptide that connects insulin's A-chain to its B-chain in the proinsulin molecule. C-peptide measurements may be made for clinical diagnostics.

As a means of distinguishing type 1 diabetes from type 2 diabetes or Maturity onset diabetes of the young (MODY).

To determine how much of their own natural insulin a person is producing as C-peptide is secreted in equimolar amounts to insulin.

C-peptide may be used for determining the possibility of gastrinomas associated with Multiple Endocrine Neoplasm syndromes.

C-peptide levels may be checked in women with Polycystic Ovarian Syndrome (PCOS) to help determine degree of insulin resistance.