Evaluation of Toxoplasma gondii IgG avidity assays through a comparison of IgM serostatus.

Primary Toxoplasma gondii (T. gondii) infection during pregnancy could result in congenital disease with severe clinical complications. IgM antibodies are one of the indices of primary infection. The IgG avidity index (AI) is also known to remain low for at least 3 months after primary infection. Here, we evaluated and compared the performance of T. gondii IgG avidity assays as confirmed by T. gondii IgM serostatus and number of days post-exposure. Four assays preferentially used in Japan were employed to measure the T. gondii IgG AI. Results for the T. gondii IgG AI showed good concordance, particularly in cases with a low IgG AI. This study confirms that the combination of T. gondii IgM and IgG AI tests is a reliable and suitable method for identifying T. gondii primary infections. Our study proposes the necessity of measuring the T. gondii IgG AI as an additional indicator of T. gondii primary infection.