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 hypo-glycemia in an insulin-dependent diabetic were attributed to surreptitious self-administration of insulin. Immuno-reactive c-peptide values were falsely increased and di-agnostically misleading when measured in unextracted plasma. After preliminary removal of antigen/antibody complexes from the plasma by extraction with polyethyl-ene glycol, the c-peptide values, referred to as ‘ ‘free c-peptide, ’ ‘ were suppressed. We suggest that insulin anti-bodies 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 mea-surements are to be accurate