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.

Incidence and mechanism of spurious increase in serum thyrotropin.

We previously reported spuriously high values for thyrotropin (TSH), presumably owing to an antibody in human serum that reacts with both reagent rabbit antibodies in an immunoradiometric assay (IRMA). We used this IRMA to measure TSH. Five of 20 sera from laboratory animal handlers showed spuriously high values. When we added 2 mL of nonimmune rabbit serum per liter to the labeled IRMA rabbit antibody reagent and reassayed the five affected specimens, the results were within the reference interval. Smaller additions partly corrected the TSH values, but nonimmune sera of eight other species had no effect. Substitution of goat solid-phase antibody decreased, but did not eliminate, the increases in TSH in three of the five affected sera. Chromatographic properties, results of rheumatoid factor testing, and measurement of human anti-rabbit immunoglobulin suggest that the interference is ascribable to an antibody of the IgG class that reacts with rabbit antibody. Evidently, antibody interference with IRMA procedures may be common in certain populations. It can be avoided by including nonimmune serum corresponding to the species used to produce reagent antibody.