C‑peptide is a single chain 31‑amino acid polypeptide that connects insulin's A-chain to its B-chain in the proinsulin molecule. C‑peptide is cleaved from proinsulin, stored in secretory granules, and eventually released into the bloodstream in amounts equimolar with those of insulin. A test of C peptide levels will show how much insulin the body is making. Measurements of C‑peptide may therefore be an aid in the assessment of a residual β‑cell function in the early stages of type 1 diabetes mellitus and for the differential diagnosis of latent autoimmune diabetes of adults (LADA) and type 2 diabetes.
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