Improved diagnosis of primary Toxoplasma gondii infection in early pregnancy by determination of antitoxoplasma immunoglobulin G avidity.

The ability to discriminate between primary Toxoplasma gondii infection acquired in early pregnancy and infection that occurred prior to pregnancy was assessed by an enzyme immunoassay (EIA) to determine the avidity of toxoplasma-specific immunoglobulin G (IgG). The results were compared to those of the Platelia Toxo-IgM EIA and the dye test. The mean IgG avidity of 73 serum samples collected within 20 weeks after the estimated time of infection was 5.9%. Among 26 serum samples showing latent infection (toxoplasma-specific IgG positive and IgM negative) and 56 IgM-positive serum samples with a low dye test titer (<300 IU/ml), the mean avidities were 51.3 and 57.5%, respectively. A total of 72.8% of 92 IgM-positive serum samples with a high dye test titer (>300 IU/ml), suggesting a recent toxoplasma infection, had an IgG avidity of >20%, indicating that the infection started more than 20 weeks earlier. By introducing high IgG avidity as a criterion in the first half of pregnancy to exclude the possibility that toxoplasma infection was acquired during gestation, many women will avoid unnecessary examinations, treatment, and anxiety.