gastrointestinal disorders
Insulinoma
Diagnosis of Insulinoma
Insulinlevel Sometimes C-peptide or proinsulin levels Endoscopic ultrasound Plasma glucose should be measured during symptoms while fasting. If hypoglycemia is present during symptoms (glucose<55 mg/dL [3.0 mmol/L]) or without symptoms (glucose<40 mg/dL [2.2 mmol/L]), aninsulinlevel should be measured on a simultaneous sample. Hyperinsulinemia of>6 mcU/mL (42 pmol/L) suggests aninsulin-mediated cause, as does a seruminsulinto plasma glucose ratio>0.3 (mcU/mL)/(mg/dL) (38.2 when using SI units [pmol/L]/[mmol/L]). Insulinis secreted as proinsulin, consisting of an alpha chain and beta chain connected by a C peptide. Because pharmaceuticalinsulinconsists only of the beta chain, surreptitiousinsulinadministration can be detected by measuring C-peptide and proinsulin levels. In patients with insulinoma, C peptide is≥0.6 ng/mL (0.2 nmol/L) and proinsulin is≥5 pmol/L. These levels are normal or low in patients with surreptitiousinsulinadministration. Because many patients have no symptoms (and hence no hypoglycemia) at the time of evaluation, diagnosis of insulinoma requires admission to the hospital for a 48- or 72-hour fast. Nearly all (98%) patients with insulinoma develop symptoms within 48 hours of fasting; 70 to 80% within 24 hours. Hypoglycemia as the cause of the symptoms is established by the Whipple triad: Symptoms occur during the fast. Symptoms occur in the presence of hypoglycemia. Ingestion of carbohydrates relieves the symptoms. Hormone levels are obtained as described above when the patient is having symptoms. If the Whipple triad is not observed after prolonged fasting and the plasma glucose after an overnight fast is>50 mg/dL (>2.8 mmol/L), a C-peptide suppression test can be done. Duringinsulininfusion (0.1 U/kg/hour), patients with insulinoma do not suppress C peptide to normal levels (≤1.2 ng/mL [≤0.40 nmol/L]). Endoscopic ultrasonography has>90% sensitivity and helps localize the tumor. Positron emission tomography (PET) also may be used. CT has not proved useful, and arteriography or selective portal and splenic vein catheterization is generally unnecessary. 