What is the role of lab testing in the diagnosis of insulinoma?

Updated: May 31, 2020
  • Author: Zonera Ashraf Ali, MBBS; Chief Editor: Neetu Radhakrishnan, MD  more...
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Failure of endogenous insulin secretion to be suppressed by hypoglycemia is the hallmark of an insulinoma. Thus, the finding of inappropriately elevated levels of insulin in the face of hypoglycemia is the key to diagnosis. Considering the reference range, the fasting plasma levels of insulin, C-peptide, and, to a lesser degree, proinsulin need not be elevated in insulinoma patients in absolute terms.

A prolonged supervised fast in hospitalized patients provides the most reliable results. During this test, levels of plasma glucose, insulin, C peptide, and proinsulin are measured every 6 hours until the plasma glucose level falls to 60 mg/dl or less, at which point the testing interval is reduced to every 1–2 h. The fast is terminated when the plasma glucose level is 45 mg/dl or less and the patient has symptoms and signs of hypoglycemia

Traditionally, a 72-hour fast has been used. However, with current assays, 90–95% of insulinomas can be diagnosed with a supervised fast of 48 hours. [15]

The calculation of ratios of insulin (µU/mL) to plasma glucose (mg/dL) during the fast is diagnostic: Healthy patients maintain a ratio of less than 0.25 (obese patients may have a slightly higher rate), whereas in patients with insulinoma, the ratio rises during fasting.

In a study from the Netherlands, a positive Whipple triad on a prolonged fasting test, 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. [26]

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