Method for differentiation of nonspecific from specfic toxoplasma IgM fluorescent antibodies in patients with rheumatoid factor.

In a study performed to define the prevalence of false positive toxoplasma IgM-IFA test results in sera containing RF, 8 (19.5%) of 41 sera which were positive for RF were positive in the toxoplasma DT and conventional toxoplasma IFA test. Three of these eight were also positive in the toxoplasma IgM-IFA test and in two, the results were considered to be false positives. Of the 33 sera remaining which were positive for RF but negative in both the DT and conventional IFA test, three were positive in the toxoplasma IgM-IFA test. Of 51 sera from patients with suspected rheumatoid arthritis or other collagen vascular disorders, all of which were negative when tested for RF, none was positive for toxoplasma IgM antibodies in the IgM-IFA test. Sera from 15 adults with the acute lymphadenopathic form of toxoplasmosis and 13 infants with congenital toxoplasmosis were tested for the presence of RF. Whereas none of the sera from the acquired cases had demonstrable RF, two of the congenital cases had RF, and their titers were both 1:320. False positive IgM-IFA test results became negative after treatment of sera with heat-aggregated IgG. In contrast, IgM-IFA test titers in cases of acute congenital or acquired toxoplasmosis were unaffected by this treatment. Thus, treatment with heat-aggregated IgG can be used to differentiate false positive IgM-IFA test titers due to RF from those due to specific IgM toxoplasma antibody.