Falsely low results in CA 125 determination due to anti-idiotypic antibodies induced by infusion of [131I]F(ab')2 fragments of the OC125 antibody.

Abstract

We investigated a one-step immunometric CA 125 assay, which employs new anti-CA 125 antibodies as capture antibodies and OC125 antibodies for detection, for interference due to antibodies induced by repeated administration of F(ab')2 fragments of the anti-CA 125 antibody OC125. Testing 33 samples, obtained from 13 patients treated with OC125 fragments, we found falsely high CA 125 concentrations only in samples with exceptionally high concentrations of both anti-idiotypic antibodies and non-specific human anti-mouse antibodies. In contrast, the recovery of added CA 125 was already diminished in the presence of low anti-idiotypic antibody concentrations. Both interferences disappeared after removal of serum IgG. It was possible to eliminate the falsely high results, but not the reduction in recovery rate, by adding non-specific murine IgG. When the binding of the detector antibodies was performed in a separate incubation step, no reduction in recovery rate was observed. Our results suggest that non-specific human anti-mouse antibodies are responsible for falsely high results. The reduction in the recovery rate is obviously due to an inhibition of the binding of OC125 detector antibodies by anti-idiotypic antibodies. In patients receiving OC125 antibodies CA 125 can be measured using OC125 detector antibodies if a two-step assay is performed. An increase in CA 125 following OC125 infusion should be confirmed after the addition of non-specific murine IgG.