C-peptide in conditions other than diabetes mellitus.

Measurement of serum and urinary C-peptide has been shown to be of value in several conditions other than diabetes mellitus. It is particularly useful because it can distinguish endogenous beta cell secretion from exogenously administered insulin and because circulating insulin-binding antibodies do not interfere with its measurement. Because the liver removes little, if any, C-peptide, peripheral blood values may more accurately reflect beta cell secretion than do peripheral insulin levels. Clinically, serum C-peptide has been most useful in diagnosing hypoglycemic disorders. Diagnosis of insulinomas is facilitated in both diabetic and nondiabetic patients, and surreptitious insulin injection is readily detected. In studies of insulin regulation, circulating C-peptide has been used to demonstrate suppression of endogenous insulin secretion by exogenous insulin. Peripheral insulin and C-peptide levels have been compared in studies of the role of the liver in states of altered insulin homeostasis. Because of its higher urinary clearance, determination of urinary C-peptide is preferable to urinary insulin measurement in situations where frequent blood sampling is impossible or difficult to accomplish.