Avidity IgG: diagnosis of primary Toxoplasma gondii infection by indirect immunofluorescent test

In the present study, 18 serum samples from toxoplasmosis patients with different clinical manifestations were classified into G1: 9 patients with suspected of having recent T. gondii infection, and G2: 9 patients with suspected of having distant latent infection. Patients with low IgG avidity antibody, 80% of them were positive by IgG avidity-indirect immunofluorescent antibody test (IIFAT) and by IgM-ELISA for toxoplasmosis. However, 20% positive by low IgG avidity- IIFAT, were IgG-ELISA positive for toxoplasmosis. Patients with high IgG avidity antibody, 15.4 % of them were positive by high IgG avidity-IIFAT, and by IgM-ELISA, but 61.5% positive by high IgG avidity-IIFAT, were positive by IgG-ELISA and 23.1% positive by high IgG avidity IIFAT, were positive with both IgM and IgG ELISA. It was concluded that detection of high avidity IgG antibodies by IIFAT excluded recent T. gondii infection, as a confirmatory test and when only a single serum sample is submitted by the patient.