Falsely elevated results of radioimmunoassays using double antibody method: arguments for a third anti-rabbit IgG antibody present in certain human sera.

Nine sera with falsely elevated results in a TSH radioimmunoassay were studied. Sera dilution and gel filtration showed that the immunoreactive TSH was different from the standard and labeled TSH. It coeluted with the immunoglobulin fraction of the serum. Rabbit serum decreased or emphasized the artifact according to the dose. Falsely elevated results appeared only in radioimmunoassays using a double antibody method for the separation of free and bound labeled hormone. FSH, LH, and beta HCG radioimmunoassays using the double antibody method underwent the same disturbance as the TSH assay in these patients. Our results suggest that the artifact may be due to the presence in certain human sera of anti-rabbit IgG antibodies, interfering in the radioimmunoassay by inhibition of the binding of first and second antibodies.