What is the gold standard test for diagnosing insulinoma?

The gold standard test for diagnosing an insulinoma is 72 hour fast test. 75% of insulinoma patients develop symptoms and blood glucose levels of less than 40mg/dl within the first 24 hour fast, 90% within 48h and 100% within 72hrs. "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 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 decrease in blood glucose levels. The clinical symptoms are because of the effects of hypoglycaemia (sweating, tremor, palpitations, confusion, headache, disorientation, visual difficulties, irrational behavior, or even coma).