Abstract
C-peptide levels are in many ways a better measure of endogenous insulin secretion than peripheral insulin levels. C-peptide may be measured in either blood or urine. Perhaps the major advantage of measuring C-peptide levels is the ability to readily distinguish endogenous insulin levels in the presence of exogenous administration of insulin. Early C-peptide immunoassays were troubled by lack of sensitivity. This problem has now been overcome, and it is possible to measure C-peptide values down to extremely low levels. The clinical indications for C-peptide measurement include diagnosis of insulinoma and differentiation from factitious hypoglycemia, follow-up of pancreatectomy and evaluation of viability of islet cell transplants. Recently these indications have been dramatically expanded to permit evaluation of insulin dependence in maturity onset diabetes mellitus.