Diagnosis of self-induced hyperinsulinism in an insulin-dependent diabetic patient by radioimmunoassay of free C-peptide.

On the basis of results of simultaneous determinations of plasma free insulin and free c-peptide, episodes of hypoglycemia in an insulin-dependent diabetic were attributed to surreptitious self-administration of insulin. Immunoreactive c-peptide values were falsely increased and diagnostically misleading when measured in unextracted plasma. After preliminary removal of antigen/antibody complexes from the plasma by extraction with polyethylene glycol, the c-peptide values, referred to as "free c-peptide," were suppressed. We suggest that insulin antibodies formed complexes with proinsulin-like material in the plasma of this patient, which accounted for most of the c-peptide immunoreactivity in her unextracted plasma. These complexes must be removed if c-peptide measurements are to be accurate.