Using dried blood spots to estimate Toxoplasma gondii seroprevalence in pregnant women in Catalonia, Spain, and to serologically diagnose congenital toxoplasmosis

Recent data on toxoplasmosis seroprevalence in Spain is limited, and pregnancy-screening programs are being discontinued. This study aimed to describe seroprevalence in pregnant women in Catalonia using newborn dried blood samples (DBS) from the Catalonian Newborn Screening (NBS) Program; as well as to assess detection of Toxoplasma gondii antibodies in DBS (Anti-Toxoplasma gondii ELISA (IgG/IgM), Euroimmun Lubeck, Germany) as a NBS tool and for retrospective diagnosis of congenital toxoplasmosis (CT) in cases from the Spanish Research Network of Congenital Toxoplasmosis (REIV-TOXO). A total of 3,231 DBS samples from NBS program (September 2022-August 2023) were randomly selected and analyzed, alongside with 30 DBS from CT confirmed cases. The overall seroprevalence of T. gondii IgG in pregnant women was 15.5% (95%CI: 14.33-16.85), higher in foreign women 22.6%, particularly Latin American (30.3%). Analysis of DBS from CT cases showed concordance in IgG detection but only 3.3% (1/30) were IgM positive despite 37% (10/27) having positive serum IgM at birth. In conclusion, IgG analysis in newborn DBS provides valuable information on pregnant seroprevalence. However, its value for identifying CT cases in retrospective diagnosis or as neonatal screening is poor given the low IgM detection. Thus, prenatal screening remains the most effective approach to identify children at risk.