Urine C-peptide is measured when a continuous assessment of pancreatic beta cell function is desired or frequent blood sampling is not practical (e.g., in children). C-peptide excretion in urine has been used to assess pancreatic function in gestational diabetes and in patients with unstable glycemic control in insulin-dependent diabetes mellitus (IDDM).
Urine C-peptide measurement is mainly to be used in patients on insulin treatment to assess endogenous insulin secretion. Its role in patients not on insulin treatment is limited.
Proinsulin is converted to insulin in the pancreatic beta cells. A by-product of this conversion is peptide C (C-Peptide, Connecting Peptide), an inactive 31 amino acid polypeptide chain. Peptide C levels are usually associated with endogenous insulin production and are not affected by exogenous (pharmaceutical) insulin administration.
C-peptide fulfills an important function in the assembly of the two-chain insulin (α- and β-chain) structure and the formation of the two disulfide bonds within the proinsulin molecule. Insulin and C-peptide are secreted in equimolar amounts and released into circulation. As half of the insulin, but almost none of the C-peptide is extracted in the liver, C-peptide has a longer half-life than insulin.
The liver does not extract C-peptide, which is removed from the circulation by the kidneys and degraded, with a fraction excreted unchanged in the urine. The concentration in urine is about 20- to 50-fold higher than in serum.