Establishing tools for early diagnosis of congenital toxoplasmosis: Flow cytometric IgG avidity assay as a confirmatory test for neonatal screening

The aim of this study was to evaluate the performance of conventional serology (Q-Preven™ and ELFAVIDAS™) and flow cytometry-based serologic tools for early serologic diagnosis of congenital toxoplasmosis. The study groups included prospectively confirmed cases of congenital toxoplasmosis (TOXO = 88) and age-matching non-infected controls (NI = 15).The results demonstrated that all samples tested positive/indeterminate for anti-T. gondii IgM screening at birth using air-dried whole blood samples. Serum samples collected at 30-45 days after birth tested positive for ELFAVIDAS™ IgG in both groups. While all NI tested negative for ELFAVIDAS™ IgM and IgA, only 78% and 36% of TOXO tested positive for IgM and IgA, respectively. Flow cytometry-based anti-T. gondii IgM, IgA and IgG reactivity displayed moderate performance with low sensitivity (47.6%, 72.6% and 75.0%, respectively). Regardless the remarkable specificity of IgG1, IgG2 and IgG3 subclasses for early diagnosis, weak or moderate specificity was observed (Se = 73.9%, 60.2% and 83.0%, respectively). The analysis of IgG avidity indices (AI) demonstrated the highest performance among the flow cytometry-based methods (Se = 96.6%; Sp = 93.3%), underscoring the low avidity index (AI < 60%) within TOXO (97.0%) in contrast with the high avidity index (AI > 60%) in NI (93%). Analysis of anti-T. gondii IgG and IgG3 reactivity for mother:infant paired samples may represent a relevant complementary tests for early diagnosis. In conclusion, a feasible high-standard algorithm (Accuracy = 97.1%) was proposed consisting of Q-Preven™ IgM screening at birth, followed by ELFAVIDAS™ IgM and flow cytometric IgG avidity analysis at 30-45 days after birth as a high performance tool for early serological diagnosis of congenital toxoplasmosis.