The interference of IgM rheumatoid factor in enzyme-linked immunosorbent assays of rubella IgM and IgG antibodies

Abstract

The interference of IgM rheumatoid factor (RF) in the indirect enzyme-linked immunosorbent assay (ELISA) for rubella IgM and IgG antibodies was evaluated quantitatively. In an ELISA for rubella IgM antibodies IgM RF produced false positive results in tests of sera with a rubella IgG concentration exceeding approximately 30 I.U./ml and an IgM RF concentration higher than 3.5 I.U./ml as determined by ELISA. Sera from 3 of 70 patients with recent rubella and sera from a similar proportion of blood donors contained IgM RF in a concentration exceeding this level. The false positive results were reproduced when an IgM RF preparation was added to sera containing rubella IgG. The rubella IgG values in ELISA, however, decreased after the addition of IgM RF . RF was absorbed by incubation of sera with a suspension of latex particles coated with aggregated human IgG. This method prevented the false positive results of the rubella IgM assay and increased the rubella IgM values of rubella convalescent sera. The activity in the rubella IgG assay also increased after absorption of RF. The interference of RF has been explained by a secondary binding of RF to rubella IgG-antigen complexes. The RF might subsequently be detected by the anti-IgM conjugate or compete with the anti-IgG conjugate. The common occurrence of IgM RF in concentrations sufficient to produce false positive results of the ELISA for specific IgM antibodies necessitates routine testing of IgM antibody positive sera for RF by a sensitive method and/or absorption of RF from such sera.