Detection of acute Toxoplasma gondii infection in pregnant women by IgG avidity and PCR analysis

During pregnancy, Toxoplasma gondii can be transmitted from mother to foetus and trigger a primary infection that may be symptomatic. It is important to distinguish between recently acquired and past infections to ensure proper treatment to minimize irreversible foetal injury. We used PCR of the B1 gene to evaluate the accuracy of T. gondii IgG antibody avidity testing in discriminating recent from past infection. In a cross-sectional study, T. gondii IgG and IgM antibodies were detected by enzyme linked fluorescence assay (ELFA) in 2120 serum samples from pregnant women referred to Karaj medical laboratories, February 2013 through March 2015 with 40 samples found positive. IgM-positive samples were evaluated by IgG avidity testing and PCR to amplify the B1 gene. Avidity studies indicated 20 samples with high IgG avidity, 15 with low IgG avidity, and five showing borderline values. The B1 gene was amplified in the borderline samples, with nine of the 15 showing low avidity. The B1 gene was not amplified in the high avidity sera. Our findings suggest that IgG avidity alone may not be sufficient to discriminate recent from past T. gondii infection and should not be used as the sole confirmatory test in pregnant women with IgG and IgM T. gondii antibodies. IgG avidity testing in combination with PCR may be more reliable for distinguishing between high- and low-risk infection and decrease the frequency of unnecessary treatment of pregnant women.