Proinsulin is converted to insulin and C-peptide in the pancreatic beta cells: the latter two peptides are secreted in equimolar concentrations. Thus, measurements of serum C-peptide provide a means of assessing pancreatic beta cell function in addition to that of insulin. This technique has proved particularly useful in insulin treated diabetic patients in whom the development of circulating insulin antibodies interferes with the radioimmunoassay of the hormone. The C-peptide assay has also been used to facilitate the diagnosis of various hypoglycemic conditions, including islet cell tumors and factitious injection of insulin. The extraction of C-peptide in the urine reflects average serum values over a period of time and urine C-peptide measurements are especially useful in children or individuals in whom repeated blood sampling is difficult.
(Arch Intern Med 137:625-632, 1977)