The gold standard test for insulinoma

"The most reliable test to diagnose insulinoma is a fast up to 72 h with serum glucose, C-peptide, and insulin measurements every 4-8 h." --Harrison An insulinoma is an endocrine tumor of the pancreas derived from beta cells that ectopically secretes insulin, which results in hypoglycemia. The clinical symptoms are because of the effects of hypoglycaemia (sweating, tremor, palpitations, confusion, headache, disorientation, visual difficulties, irrational behaviour, or even coma) The diagnosis of insulinoma requires the demonstration of an elevated plasma insulin level at the time of hypoglycemia. This is because insulinomas do not reduce the secretion of insulin in the presence of hypoglycaemia. Whereas in a normal person, insulin levels would decrease with a decrease in blood glucose levels. The most reliable test to diagnose insulinoma is a fast up to 72 h with serum glucose, and insulin measurements every 4-8 h. The test is terminated if the patient becomes symptomatic or glucose levels are <40 mg/dL (2.2 mmol/L). Diagnosis of insulinoma is made if: A serum insulin level of 6 microunit/mL or more in the presence of blood glucose values below 40 mg/dL the ratio of plasma insulin to glucose is > 0.3 In addition to the above criteria, some other measurements are also required before making the diagnosis of insulinoma C-peptide level serum proinsulin level, serum sulfonylurea levels These additional tests are used because any hidden use of insulin or oral hypoglycemics taken by the patient would resemble insulinoma causing high levels of insulin in presence of hypoglycaemia. The combination of elevated proinsulin levels (normal in exogenous insulin/hypoglycemic agent users), elevated C-peptide levels (low in exogenous insulin users), antibodies to insulin (positive in exogenous insulin users), and measurement of sulfonylurea levels in serum or plasma will allow the correct diagnosis to be made. Ref : Harrison 17/e p2354