The diagnosis of insulinoma is suspected in a patient with symptomatic fasting hypoglycemia. The conditions of
Whipple’s triad need to be met for the diagnosis of "true hypoglycemia" to be made: • symptoms and signs of hypoglycemia, • concomitant plasma glucose level of 45 mg/dL (2.5 mmol/L) or less, and • reversibility of symptoms with administration of glucose.
Blood tests These blood tests are needed to diagnose insulinoma: • low
glucose levels • elevated
insulin levels • elevated
C-peptide levels If available, a
proinsulin level might be useful, as well. Other blood tests may help rule out other conditions which can cause hypoglycemia.
Suppression tests Normally, endogenous insulin production is suppressed in the setting of hypoglycemia. A 72-hour fast, usually supervised in a hospital setting, can be done to see if insulin levels fail to suppress, which is a strong indicator of the presence of endogenous hyperinsulinemia, of which the most common cause is insulinoma (followed by autoimmune causes). During the test, the patient may have calorie-free and caffeine-free liquids. Capillary blood glucose is measured every 4 hours using a
reflectance meter, until values < 60 mg/dL (3.3 mmol/L) are obtained. Then, the frequency of blood glucose measurement is increased to every hour until values are < 49 mg/dL (2.7 mmol/L). At that point, or when the patient has symptoms of hypoglycemia, a blood test is drawn for serum glucose, insulin, proinsulin, and C-peptide levels. The fast is then stopped at that point, and the hypoglycemia is corrected with intravenous dextrose or carbohydrate-containing food or drink.
Diagnostic imaging The insulinoma might be localized by noninvasive means, using
ultrasound,
CT scan, or
MRI techniques. An indium-111 pentetreotide scan is more sensitive than ultrasound, CT, or MRI for detection of somatostatin receptor positive tumours, but not a good diagnostic tool for insulinomas. An
endoscopic ultrasound has a sensitivity of 40-93% (depending on the location of the tumour) for detecting insulinomas. Sometimes,
angiography with percutaneous transhepatic
pancreatic vein catheterization to sample the blood for insulin levels is required.
Calcium can be injected into selected arteries to stimulate insulin release from various parts of the pancreas, which can be measured by sampling blood from their respective veins. The use of calcium stimulation improves the specificity of this test.During surgery to remove an insulinoma, an intraoperative ultrasound can sometimes localize the tumour, which helps guide the surgeon in the operation and has a higher sensitivity than noninvasive imaging tests. ==Treatment==