Comparison of two commercially available avidity tests for toxoplasma-specific IgG antibodies.

Background: IgG anti-Toxoplasma avidity tests have proven useful to discriminate between recent and distant infection in management of pregnant women with anti-Toxoplasma-IgM antibodies. This study was performed to compare two commercially available IgG anti-Toxoplasma avidity test kits.

Methods: Sixty-four samples positive for IgM and IgG anti-Toxoplasma antibodies were analyzed with two IgG anti-Toxoplasma avidity tests: VIDAS Toxo IgG avidity kit (BioMérieux, Marcy-l'Etoile, France) and Toxoplasma gondii IgG avidity EIA kit (Labsystems, Helsinki, Finland). Samples were obtained from both pregnant women and subjects in different clinical settings.

Results: Of 64 sera positive in both VIDAS Toxo IgG and VIDAS Toxo IgM tests, 19 (29.7%) had high avidity antibodies in VIDAS and 17 (26.6%), high avidity antibodies in Labsystems kit. A total of 48 (75.0%) of 64 sera showed concordant results between VIDAS and Labsystems Toxo IgG avidity tests; one (1.2%) serum sample showed discordant results and in 15 (23.4%) sera, results were high or low in one test and borderline in the other. Overall correlation coefficient for results obtained in both tests was 0.80 (0.60 in sera of pregnant women and 0.88 in sera of other patients, respectively).

Conclusions: Correlation between results obtained in BioMérieux and Labsystems Toxo IgG avidity kits in sera from pregnant women as well other individuals was high. BioMérieux assay possesses the advantage of automatization but requires the VIDAS machine; in contrast, Labsystems assay is less costly but requires time-intensive manual performance and calculation of test results.