Evaluation of a new assay for the detection of IgM in recent and congenital Toxoplasma infections

Toxoplasma gondii infection is usually asymptomatic in immunocompetent individuals. However, primary infection during pregnancy can lead to severe fetal manifestations. Consequently, accurate diagnosis of maternal infection is essential for prompt treatment. Currently, diagnosis mainly relies on detection of anti- T . gondii specific IgM and IgG in serum. If detection of IgM can be indicative of acute infection, it may also reflect the presence of persistent or non-specific antibodies. The Toxo-ISAGA IgM assay (bioMérieux, France), formerly considered a gold standard for anti- T . gondii IgM detection, was discontinued in 2024. New and reliable assays are, therefore, needed to support risk assessment and clinical decision-making, particularly in maternal and neonatal settings. A new generation of the VIDAS TOXO IgM assay (bioMérieux, France), one of the most commonly used commercially available assays for the detection of anti- T. gondii specific IgM in serum, has recently been developed: the VIDAS TXMN assay (reference 424641). This study aimed at evaluating its clinical performance. A retro-prospective study was conducted in two French clinical parasitology–mycology laboratories in 2024. Performance was determined on 1,275 single sera from patients with recent or chronic postnatal Toxoplasma infection and non-infected patients and on serial serum samples from both women who seroconverted during pregnancy and neonates with and without congenital Toxoplasma infection. In addition, qualitative results were compared to those obtained with frequently used commercially available comparators and ISAGA. Sensitivity and specificity of the VIDAS TXMN assay were 98.77 (95% confidence interval [CI]: 93.31–99.97%) and 100.00% (95% CI: 99.59–100.00%), respectively. It allowed earlier detection of IgM than the previous generation assay both in pregnant women and in newborns from mothers who seroconverted during pregnancy. In addition, agreements with the Architect Toxo IgM (Abbott Diagnostics), Platelia Toxo IgM (Bio-Rad), and Toxo-ISAGA (bioMérieux) were almost perfect (Cohen’s kappa > 0.85). The new VIDAS TXMN assay improved the detection of anti- T. gondii -specific IgM in serum compared to the previous generation and displayed high agreement with commercially available comparators and ISAGA. It, therefore, appears to reliably contribute to the diagnosis of maternal and congenital toxoplasmosis. IMPORTANCE Toxoplasma gondii is an ubiquitous parasite, which can cause severe fetal damage when first contracted in a pregnant woman. Serology—and particularly the detection of anti- T. gondii -specific IgM antibodies—plays a key role in diagnosing recent infections and, therefore, contributes to appropriate management of pregnant women and newborns. Commercial availability of anti- T. gondii -specific IgM assays has recently evolved, with the discontinuation of the Toxo-ISAGA IgM assay (bioMérieux, France) as a gold standard and the marketing of a new version of the VIDAS TOXO IgM assay (bioMérieux, France). This study first establishes the performance of this new assay for diagnosing maternal and congenital infections, with a direct comparison with other commonly used commercial assays. This evaluation is essential for ensuring reliable diagnosis and supporting clinical decision-making, particularly in maternal and neonatal settings.