A glucose reduction challenge in the differential diagnosis of fasting hypoglycemia: a two-center study.

Investigation of patients with suspected or proven hypoglycemia is often a time-consuming and expensive process. We describe a glucose reduction challenge test which may be useful as an out-patient screening procedure. Insulin is infused for 3 h at 40 mU/kg.h. Plasma glucose was monitored at the bedside during the test, and blood samples were collected for measurement of C-peptide. Responses were examined in 17 normal controls, and 6 patients with insulinomas. In normal subjects, mean plasma glucose fell to a plateau value of 3.2 +/- 0.2 mmol/L (57 +/- 2.6 mg/dL) and remained at that level with few symptoms. In contrast, five of six patients with insulinomas developed severe hypoglycemia, with plasma glucose levels between 1.9 (34 mg/dL) and 2.2 mmol/L (39 mg/dL). Plasma C-peptide concentrations were suppressed to 0.08 pmol/mL or less in normal subjects, but in insulinoma patients remained at 0.32-1.6 pmol/mL i.e. outside the normal range, and diagnostic of nonsuppressible insulin secretion. These data demonstrate that moderate reduction of serum glucose maintained for a prolonged period results in marked suppression of plasma C-peptide, permitting improved discrimination between normal subjects and patients with insulinomas. This glucose reduction challenge can, therefore, be used as a test of glucose-regulating ability, where failure (hypoglycemia) per se represents a measurable abnormality. C-Peptide measurements will determine whether the cause of hypoglycemia is due to hyperinsulinemia.