Insulinoma laboratory tests

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]Associate Editor(s)-in-Chief: Parminder Dhingra, M.D. [2]

Overview

Laboratory findings consistent with the diagnosis of insulinoma include 72 hour suppression test and Whipple's triad.

Laboratory Tests

Laboratory findings consistent with the diagnosis of insulinoma include:

Whipple's Triad

The diagnosis of insulinoma is suspected in a patient with symptomatic fasting hypoglycemia. The conditions of Whipple’s triad that need to be met for the diagnosis of insulinoma are:

1. Hypoglycemia (plasma glucose < 50 mg/dL)

2. Neuroglycopenic symptoms

3. Reversibility of symptoms with administration of glucose

Suppression Tests

The gold standard blood tests needed to diagnose insulinoma during a 72 hour fast are:

• Plasma glucose level
• Insulin
• C-peptide
• Pro-insulin

The sensitivity of combination of these 4 tests after 72 hour fasting is around 99%.

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 an insulin-secreting tumor. 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 stopped at that point, and the hypoglycemia is treated with intravenous dextrose or calorie-containing food or drink.

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