Background: Insulinomas are rare, but are the most common cause of hyperinsulinemic hypoglycemia in the adult population. Diagnosis of this pathology relies on clinical features along with laboratory tests and imaging investigations to aid in localization. One of the most robust standard tests used for establishing a biochemical diagnosis is the prolonged (72-hour) fast. Currently, it is recommended that a prolonged supervised fast be performed, at least for 48 h if not for 72 h, and many would take the absence of hypoglycemia after a 72-hour fast as evidence excluding the diagnosis.

Methods: Prolonged fasts and standard glucose tolerance tests, plus imaging studies and surgical pathology, in two patients with suspected insulinomas.

Results: The prolonged 72-hour fast was normal in both the patients, while in both cases a prolonged oral glucose tolerance test clearly demonstrated the induction of severe hyperinsulinemia followed by significant hypoglycemia. Surgical removal confirmed the presence of insulinomas in each case.

Conclusions: Although the sensitivity of the 72-hour fast is high and still plays an important role in the diagnosis of an insulinoma, we suggest that a "normal" test should be interpreted in the light of clinical symptoms.