The page below is a sample from the LabCE course Diabetes: Diagnosis, Laboratory Testing, and the Current American Diabetes Association Guidelines. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Insulin and C-Peptide

Insulin is secreted by the pancreatic beta-cells as a prohormone composed of fragments: C-peptide and insulin. The C-peptide fraction is cleaved off the prohormone. The insulin fraction becomes active. C-peptide is inactive but provides structure to the prohormone and has a much longer half-life. Both of these hormones can be quantitated in blood.
Insulin levels are not measured to diagnose or monitor diabetes, but can give information about a patient and is an important assay in low blood sugar (hypoglycemia). C-peptide is also measured in evaluating hypoglycemia and is used to distinguish between endogenous and exogenous insulin; it would be present in circulation in endogenous insulin secretion.  In addition, it is also often used to monitor pancreatic surgery and transplant because of its longer half-life.