Comparative IgG-IgM Western Blot in the Diagnosis of Congenital Toxoplasmosis: A Valid Diagnostic Tool

Congenital toxoplasmosis (CT) results from vertical transmission of <i>Toxoplasma gondii</i> during maternal infection in pregnancy. Early diagnosis in newborns is crucial to initiate timely therapy and prevent long-term sequelae. The IgM Immunosorbent Agglutination Assay (ISAGA) has historically been considered an important diagnostic tool for CT; however, its recent market withdrawal necessitates alternative approaches. We conducted a retrospective observational study at Fondazione IRCCS Policlinico San Matteo, Pavia, Italy, including 44 newborns born to mothers with confirmed toxoplasmosis between 2019 and 2022. Newborns were classified as CT (n = 19) or non-CT (n = 25) based on serological follow-up, comparative Western blot (CWB) and Interferon Gamma Release Assay (IGRA). Sensitivity and specificity of CWB, IgM Chemiluminescent Immunoassay (CLIA), and IgM ISAGA were assessed at birth and at one month. At birth, CWB demonstrated 88.9% sensitivity, significantly higher than IgM CLIA (52.6%) and IgM ISAGA (57.9%). Specificity was 100% at birth and 92% at one month. CWB retained high sensitivity at one month (81.8%). IGRA complemented CWB in confirming or excluding infection in cases with equivocal or false-negative serology. Comparative Western blot thus represents a robust diagnostic alternative for CT, ensuring early detection and timely treatment, particularly in the absence of IgM ISAGA.