This test measures the level of C-peptide in a blood. Proinsulin is the inactive precursor of insulin and is synthesized in the beta cells of the pancreas. Cleavage of proinsulin produces insulin and C-peptide, which are stored in the beta cells. When insulin is required and released into the bloodstream to help transport glucose into the body’s cells (to be used in energy production), equal amounts of C-peptide also are released. This makes C-peptide useful as a marker of insulin production.

C-peptide can be used to help evaluate the production of endogenous insulin (insulin made by the body) and to help differentiate it from exogenous insulin (insulin that is not produced by the body and so does not generate C-peptide).