Gold standard test for the diagnosis of insulinoma is a supervised 72 hour fast test with measurement of serum glucose, C-peptide, proinsulin, and insulin every 4-8 hour. In non obese normal subjects, serum insulin levels should decrease to <6 U/mL when blood glucose decreases to <40 mg/dL and the ratio of insulin to glucose is <0.3 (in mg/dL). In a case of insulinoma, level of insulin increases >6 U/mL when blood glucose is <40 mg/dL. Other tests positive in insulinoma are an elevated C-peptide and serum proinsulin level, an insulin/glucose ratio >0.3, and a decreased plasma-hydroxybutyrate level. Ref: Harrison's Internal Medicine, 18th Edition, Chapter 350