An improved assay for the detection of Toxoplasma gondii antibodies in human serum by flow cytometry.

The diagnosis of toxoplasmosis relies primarily on the demonstration of specific antibodies to Toxoplasma gondii. We describe a flow cytometry method for the determination of antibodies to whole fixed tachyzoites by indirect immunofluorescence. Fixed tachyzoites in suspension have characteristic light scattering properties. The amount of IgM, IgG, and IgA antibodies from patients' sera bound to the tachyzoites can be estimated from the mean fluorescence intensities observed using class-specific conjugates for different Ig heavy chains. Appropriate serum dilutions for the estimation of specific IgG titres and for the discrimination between sera positive and negative for IgA and IgM antibodies were established using 40 random sera from pregnant women. The method proved to be quantitative and highly sensitive as compared with currently used assays. Coefficients of variation between series ranged from 6.4% to 12.2% and could be controlled by the inclusion of positive and negative standard sera in the test series. The method might represent a major improvement in cost effectiveness as compared with routine immunofluorescence assay by fluorescent microscopy when the volume of testing is sufficient. It can be fully automated, and its large scale application is currently under investigation.