An enzyme immunoassay for immunoglobulin M antibodies to Toxoplasma gondii which is not affected by rheumatoid factor or immunoglobulin G antibodies.

An enzyme-linked immunosorbent assay (ELISA) for total antibodies to Toxoplasma gondii was modified to measure specific immunoglobulin M (IgM) antibodies. The assay requires three incubation periods totaling 2 h and enzyme-labeled-heavy-chain-specific antibodies to human IgM. The objective read-out in absorbance was normalized to percent of a standardized positive control for interpretations. No difference was observed between the assay results with or without previous absorption of the samples by Staphylococcus aureus protein A to remove most of the IgG antibodies. Addition of serum containing very high levels of IgG antibodies to another containing both IgG and IgM antibodies did not change the IgM assay values for the latter. None of the 22 sera containing high levels of IgM rheumatoid factor (RF) gave positive ELISA IgM results, even though 8 of them also had high levels of IgG toxoplasma antibodies. Mixtures of sera containing high concentrations of RF with sera having high levels of IgG toxoplasma antibodies also failed to show any false-positive reactions in the IgM toxoplasma assay. Thus, this ELISA for T. gondii IgM antibodies was not affected by IgG toxoplasma antibodies and RF.