Urine C-peptide is measured when a continuous assessment of pancreatic beta cell function is desired or frequent blood sampling is impractical (e.g., in children). C-peptide excretion in urine has been used to assess pancreatic function in gestational diabetes and patients with unstable glycemic control in insulin-dependent diabetes mellitus (IDDM).
Urine C-peptide measurement is mainly used to assess endogenous insulin secretion in patients on insulin treatment. Its role in patients who are not on insulin treatment is limited.
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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 unaffected by exogenous (pharmaceutical) insulin administration.
C-peptide is vital in assembling the two-chain insulin (α- and β-chain) structure and forming 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 degraded by the kidneys from circulation. A fraction is excreted unchanged in the urine, where the concentration is about 20—to 50-fold higher than in serum.
