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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