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A by-product of the manufacture of insulin within the beta cells of the pancreas. Insulin is produced from a protein called proinsulin, which consists of three chains of amino acids: the A-chain, the connecting peptide (or C-peptide) chain, and the B-chain. Within the islets, the C-peptide breaks away and the A-chain and B-chain combine to make insulin. The C-peptide is secreted into the bloodstream along with the insulin.

C-peptide levels can be measured using a simple blood test. They are sometimes measured in people with Type 2 diabetes to determine whether any insulin is still being produced by the pancreas. (Injected insulin contains no C-peptide.) Some insurance providers, including Medicare, require either a C-peptide test or a test for the presence of beta-cell antibodies before providing coverage for insulin pumps. A person whose C-peptide level falls within the normal, nondiabetic range (0.5–3.0 nanograms per milliliter) is generally not eligible for insulin pump coverage in these insurance plans.

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