Interpretation of the Fasting Test

Plasma Insulin Concentrations. A plasma insulin concentration of 6 (iU/mL (36 pmol/L) by radioimmunoassay (or 3 (iU/mL by immunometric assay) when the plasma glucose concentration is below 45 mg/dL (2.5 mmol/L) indicates an excess of insulin and is consistent with insulinoma. Unfortunately, plasma glucose concentrations fall below 50 mg/dL (2.8 mmol/L) in some normal subjects and remain above 50 mg/dL in an occasional patient with an insulinoma.16

Plasma C Peptide Concentrations. It is essential to measure plasma C peptide at the end of the fast; measurement of plasma proinsulin can also be helpful. Measurements of plasma C peptide distinguish endogenous from exogenous hyperinsulinemia. In patients in whom plasma glucose concentrations fall below 45 mg/dL (2.5 mmol/L), there is no overlap in the values in insulinoma patients and normal subjects at a plasma C peptide value of 0.2 nmol/L. All insulinoma patients have higher values and all normal subjects who are hypoglycemic have lower values. For plasma proinsulin, the diagnostic criterion for insulinoma is 5 pmol/L or greater.16

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