Endocrine Abstracts

Introduction: Insulinomas are rare pancreatic NETs presenting with chronic hypoglycaemia. Current guidelines for diagnosis require a prolonged fasting test (72 h), which implies hospitalization and is difficult to perform, delaying prompt diagnosis and treatment. It has been reported that 65 to 85% of insulinomas could be diagnosed after less of a 24h fasting period and 94 to 95.7% within 48h, although a shorter test for diagnosis has not yet been standardized.

Objective: To predict weather a shorter outpatient fasting test initiated overnight and portrayed up until 24h could be a sensitive method for diagnosing insulinoma.

Materials & Methods: we conducted a retrospective monocentric study to evaluate the diagnostic performance of the short fasting test to achieve the diagnosis of insulinoma. All subjects admitted from 2019 to 2021 at the Unit of Endocrinology of the Sant’Andrea Hospital with clinical suspicion for insulinoma (documented hypoglycaemia in absence of other known causes or intake of hypoglycaemic drugs) underwent the short fasting test (n=9). A comparison study was performed with subjects who underwent the standard prolonged fasting test from 2003 to 2018 (n=22). The short fasting test is initiated by the patient overnight at home and proceeds the following day in outpatient setting (Day Hospital). As in the standard protocol, symptoms and capillary blood glucose (CBG) are strictly monitored. Venous blood is drawn for glycaemia, insulin and C-Peptide at admission, in case of symptoms of hypoglycaemia or if CBG ≤45 mg/dl. Diagnostic values for insulinoma consist of glucose < 55 mg/dl with insulin ≥ 3μU/ml and C-Peptide ≥ 0.6 ng/ml. In case of a negative result and a remaining high suspicion for insulinoma, the prolonged test would be performed subsequently.

Results: The final sample consisted of 31 patients, with mean age ± Standard Deviation (SD) of 44.5 ± 12.6 years (17-74). Diagnostic values for insulinoma were found in a total of 10 patients: in 6/22 who underwent the prolonged test and in 4/9 who underwent the short fasting test. Time counting from the last meal until diagnosis ranged from 4h to 30h, with average ± SD of 11 ± 7.3 h; however, only one patient showed diagnostic values at >24h (30h).

Conclusion: In our series, 9/10 (90%) patients with insulinoma were diagnosed within 14 h from the beginning of the fast. A short fasting test could be a valid, sensitive and reliable first-line workup in diagnosing insulinoma, without hospitalization.