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Patients with insulinoma may exhibit signs and symptoms of hypoglycemia. Plasma glucose levels are low (<45 mg/dL), but fasting insulin levels may be within the normal range. Insulin assays also cannot distinguish between endogenously secreted and factitious exogenous insulin injection.

Proinsulin measurement is a useful adjunct for the diagnosis of insulinoma. Proinsulin is normally converted to insulin and C-peptide in the beta cells. Hypersecretion of proinsulin by a tumor overwhelms this proteolytic pathway, resulting in the release of proinsulin into the circulation. Proinsulin levels are typically greater than 20 pmol/L.

Patients with chronic renal failure and Type 2 diabetes may have increased proinsulin values but do not have concomitant hypoglycemia.

Reference range for a fasting specimen is 3 - 20 pmol/L.

Specimen requirement is one lavender top (EDTA) tube of blood collected from a fasting patient and immediately placed in ice. The tube should be immediately centrifuged and the plasma separated and frozen in a plastic vial.

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