C-Peptide, Fasting

C-peptide is a short chain of amino acids released into the blood as a by-product of insulin production. This test measures the amount of C-peptide in blood and helps evaluate the production of insulin made by the body (endogenous). This helps in differentiating endogenous insulin from the insulin that is not produced by the body but is taken in as diabetic medication (exogenous). This is possible because the insulin that is taken in as diabetic medications does not generate C-peptide.

Beta cells of the pancreas produce proinsulin which is a biologically inactive molecule. This molecule splits apart to form one molecule of C-peptide and one molecule of insulin. The uptake, utilization, and storage of glucose after it is absorbed in the intestines is facilitated by the hormone called insulin. When insulin is released from the beta cells into the blood in response to increased levels of glucose, equal amounts of C-peptides are also released. Since C-peptide is produced at the same rate as insulin, it is useful as a marker of insulin production.