The IgG avidity value for the prediction of Toxoplasma gondii infection in the amniotic fluid

Primary Toxoplasma gondii (T. gondii) infection during pregnancy may lead to congenital toxoplasmosis. Maternal screening using T. gondii IgG avidity measurement and multiplex nested PCR was performed. The aim of this prospective cohort study was to determine a cut-off value of IgG avidity index (AI) for the prediction of the presence of T. gondii DNA in the amniotic fluid. One hundred thirty-nine women with positive or equivocal tests for IgM underwent both serum IgG avidity measurement and PCR analysis for the amniotic fluid. Nine had positive PCR results, and three of them were diagnosed as having congenital infection. A cut-off value of IgG AI was determined using receiver operating characteristic analysis. IgG AI (mean 13%) in women with positive PCR results was significantly lower than that (39%) in women with negative results. A cut-off value of <25% IgG AI yields the best results with 77.8% sensitivity and 81.5% specificity for the presence of T. gondii DNA in the amniotic fluid. None of women with IgG AI of ≥30% had a positive PCR result or congenital infection. This study firstly demonstrated that a cut-off value of 25–30% IgG AI might be useful for the prediction of the presence of T. gondii DNA in the amniotic fluid and congenital infection.