Discrepancies between a new highly sensitive Toxoplasma gondii ELISA assay and other reagents: interest of Toxo IgG Western blot.

Immunodiagnostic assays are commonly used to screen for maternal toxoplasmic seroconversion during pregnancy. The introduction to the market of a new highly sensitive IgG assay, the Elecsys Toxo IgG test, has resulted in discrepancy issues with other immunoassays because of a lack of standardisation. Western blot appears to be a good alternative gold standard to the dye test, as the latter is not routinely available. For the present prospective study, we compared the analytical performances of two immunoassays, Elecsys Toxo IgG (Roche Diagnostics) and Platelia Toxo IgG (Bio-Rad, Marnes la Coquette, France), to Toxo II IgG Western blot (LDBio, Lyon, France) using 231 consecutive sera with low or equivocal IgG titres. Of these 231 sera, 213 presented discrepancies, which showed the importance of a confirmation test. Of the Elecsys Toxo IgG-positive results, 100% were confirmed by the Western blot with a positive threshold of 30 IU/ml for Elecsys; in the equivocal area (1-30 IU/ml), Western blot is negative in 54% of cases. Our results suggest that the lower diagnostic cut-off of Platelia Toxo IgG should be further reduced. Our study indirectly confirms that monitoring, especially for pregnant women, must be done in the same laboratory using the same technique. The ability to diagnose very early seroconversion using Western blot merits further study.