Circulating human antibodies as a cause of falsely depressed ferritin values

We have previously reported spuriously elevated values for serum thyrotropin (TSH) measured by immunoradiometric assay (IRMA). We tested those sera showing interference with the TSH assay in a ferritin IRMA which used 125I-rabbit antiferritin antibody and solid phase goat antiferritin antibody as reagents and spleen ferritin as standard. In this assay, we obtained falsely low ferritin values that were corrected by addition of 0.2% non-immune rabbit serum to the labelled antibody. Two other radioimmunoassays gave results with these sera that were not falsely lowered. The interference was shown to be due to human IgG reactive with rabbit serum, the specificity of the interfering antibody being similar in all affected sera. Human antibodies directed against immunoassay reagents may lead to spuriously increased or decreased immunoassay results depending on the specific reagents involved.