Interference of Immunoglobulins in the Radioimmunoassay

Summary: The interference of immunoglobulins in the radioimmunoassay (RIA) of human ß-endorphin was investigated. Human IgM showed no cross-reactivity. Human IgA showed a weak cross-reaction, but the dilution curve of IgA did not show parallelism with the standard curve of ß-endorphin, thus indicating its antigenic difference. The dilution curves of human IgG showed 0.18 % displacement with respect to the human ß-endorphin standard curve, with good parallelism. Moreover, five patients with multiple myeloma of the IgG type showed falsely elevated ß-endorphin levels. We investigated the possibility that certain IgGs may be responsible for the displacement of [125I] ß-endorphin in the ß-endorphin kit. The apparent ß-endorphin level of plasma from multiple myeloma patients was markedly decreased after affinity chromatography of the serum on protein A-Sepharose. In another 3 patients with multiple myeloma, we examined IgG interference by measuring the ß-endorphin levels in their lyophilized IgG diluted with saline. The results demonstrated high values of 20.2, 25.5 and 21.2 pmol/1 respectively, also showing good parallelism. These immunological parallels to human ß-endorphin verify that a part of the amino acid sequence of human IgG is similar to that of human ß-endorphin. Consequently, in the measurement of ß-endorphin with polyclonal antibody, the results may sometimes be spuriously high due to cross-reaction with IgG, e. g. in patients with IgG myeloma. To avoid IgG interference