Evaluation of insulin secretory capacity in diabetes mellitus

The capacity of the pancreatic beta cells to secrete insulin can be evaluated in vivo by measurement either of circulating immunoreactive insulin or of C peptide, a by-product of insulin synthesis. Evaluation of serum levels is complicated, however, by the variable degradation rates and distribution spaces of these peptides. Also, interpretation of peripheral vein concentrations of insulin, and perhaps C peptide, is more difficult because of hepatic catabolism. Quantitation of these peptides in the urine may provide an integrated measure of insulin secretion. In insulin treated diabetic patients who develop circulating insulin antibodies, beta cell secretory capacity may be assessed by C peptide measurement, or by techniques that allow separate determinations of "free" and "total" insulin. A variety of stimulatory tests may be used to investigate insulin secretory capacity.