Immunoreactive insulin assays do not always indicate the serum concentration of insulin secreted by the patient's pancreas. In insulin-treated diabetic patients, circulating endogenous insulin antibodies compete with antibodies for the added labeled hormone. The immunoassay may then fail to detect a coexisting insulinoma or to diagnose a factitious hypoglycemia induced by surreptitious insulin injections. In these situations a "collateral," less direct approach such as measurement of correlated connecting (C)-peptide concentration may be more revealing.
C-peptide, which forms the connecting link between alpha and beta chains of the proinsulin molecule, is released from the beta cells into the circulatory system when proinsulin is converted into insulin. Secreted in equimolar quantities with insulin, the peptide follows a different metabolic route, but retains a reproducible correlation with insulin. Its determination thus provides a truer reflection of endogenous insulin concentration than the insulin radioimmunoassay. Used as an index of beta-cell function, the response of