Evaluation of endocrine tests. D: the prolonged fasting test for insulinoma.

OBJECTIVE To establish the diagnostic performance of the prolonged fasting test in patients suspected of insulinoma. METHODS We included all patients who were referred to our department between August 1995 and August 2006 with a clinical suspicion of insulinoma. Insulinoma was diagnosed by a positive Whipple's triad during the prolonged fast in combination with an insulin/C-peptide ratio below 1. The presence of insulinoma was confirmed by histopathological data, which was considered the golden standard. If the prolonged fast was negative, long-term follow-up was obtained. RESULTS Ten patients had a positive Whipple's triad during the prolonged fast: eight had a histologically proven insulinoma, and two had factitious hypoglycaemia (insulin/C-peptide ratio >1.0) One additional patient likely had an insulinoma, but the Whipple's triad remained absent at up to 56 hours of fasting. Follow-up (median 53 months (3 to 142) in 76% of patients with a negative fasting test revealed no missed cases of insulinoma. During the prolonged fast the glucose, insulin and C-peptide concentrations overlapped in patients with and without insulinoma. CONCLUSION In our centre, the prolonged fasting test defined as a positive Whipple's triad in combination with an insulin/C-peptide ratio <1 had a sensitivity of 88.9% and a specificity of 100% for the diagnosis of insulinoma.