Evaluate insulin producing neoplasm (islet cell tumor, insulinoma) or pancreatic islet cell hyperplasia; work-up for hypoglycemia to evaluate insulin production in diabetes mellitus. Insulin-secreting tumors usually present with hypoglycemic symptoms and are diagnosed by a 72-hour prolonged fast. After 72 hours of fasting an insulin concentration greater than 6 mU/L indicates an insulinoma. Factitious hypoglycemia from exogenous insulin shows increased insulin, but not proinsulin or C-peptide.