Journal of Clinical Endocrinology & Metabolism, Vol 78, 1048-1051, Copyright © 1994 by Endocrine Society

ARTICLES

Proinsulin by immunochemiluminometric assay for the diagnosis of insulinoma

PC Kao, RL Taylor and FJ Service
Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, Minnesota 55905.

We used a newly developed immunochemiluminometric assay of proinsulin to determine its relative utility vis-a-vis C-peptide and insulin for the diagnosis of insulinoma. The evaluation was conducted in 20 consecutive patients with histologically confirmed insulinoma and 22 normal subjects who underwent a prolonged fast according to a standard protocol. Patients with insulinoma fasted to the point of demonstrating Whipple's triad; normal subjects fasted to 72 h. At the end of the prolonged fast, when the glucose value was 2.8 mmol/L or less (50 mg/dL), all three hormones had equal sensitivity (100%) in detecting insulinoma with no overlap with the values of normal subjects. When glucose levels were between 2.8 mmol/L (50 mg/dL) and 3.3 mmol/L (60 mg/dL) at the end of the prolonged fast, proinsulin was better than C- peptide and insulin in the diagnosis of insulinoma. The sensitivity was 90% for proinsulin and 85% for both C-peptide and insulin. Therefore, proinsulin not only is useful for the diagnosis of insulinoma, but it may have greater diagnostic accuracy than C-peptide and insulin.

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