Will we miss specific IgA detection for the diagnosis of congenital toxoplasmosis? A French retrospective monocenter study on 483 infants

To estimate the performance of anti-<i>Toxoplasma</i> IgA assay for the diagnosis of congenital toxoplasmosis, a retrospective monocenter study was conducted comparing serological results obtained in the framework of routine diagnosis workup. All infants born to mothers infected with <i>Toxoplasma gondii</i> during pregnancy from 2010 to 2023 with at least 6 months of serological follow-up were included. Four hundred and eighty-three cases (1,171 sera) were included, of which 56 infants (11.6%) were infected. Twenty out of 60 infants (33.3%) with positive IgA were not infected. The sensitivity to detect IgA antibodies in infected neonates was 71.4%. The specificity was 97.3%. The mean time to detect IgA was 7.7 ± 13.0 days in infected neonates. Anti-<i>Toxoplasma</i> IgA was the earliest positive serological test in only two cases (5.0%) but turned negative at 1 month in the absence of specific treatment, suggesting non-specific detection. IgA was associated with anti-<i>Toxoplasma</i> IgM, neosynthetized IgG or IgM on comparative Western blotting (WB), or both IgM and neosynthetized IgG or IgM on WB, in 12 cases (30.0%), 2 cases (5.0%), and 19 cases (47.5%), respectively. Sixteen infants with no IgA after birth were diagnosed via neosynthetized IgG or IgM on comparative WB and/or IgM (<i>n</i> = 10), or via PCR on amniotic fluid (<i>n</i> = 5), or persistent IgG (<i>n</i> = 1). Our study suggests that anti-<i>Toxoplasma</i> IgA is not a critical serological parameter for the diagnosis of congenital toxoplasmosis, and the recent withdrawal of commercialized reference anti-<i>Toxoplasma</i> IgA assays should not affect patient care.IMPORTANCEThis study will help clinical microbiologists estimate the impact of the withdrawal of anti-<i>Toxoplasma</i> IgA reference assays for the diagnosis of congenital toxoplasmosis and will contribute to actualize the recommendations for laboratory diagnosis.