Toxoplasma gondii (T. gondii) infection is a common protozoan parasitic infection in humans, and can be caused by ingesting of raw and undercooked meat that contains cysts or consuming oocyst-contaminated water, food, and soil. During pregnancy, vertical transmission from mothers with primary T. gondii infection to fetuses leads to congenital T. gondii infection of their infants, which manifest as retinochoroiditis, intracranial calcification, hydrocephalus, and developmental retardation.

Serum IgG and IgM antibody measurements are usually performed to diagnose acute T. gondii infection. However, T. gondii IgM antibody sometimes persists for years after the acute infection [1,2], and consequently, a false-positive result often occurs during pregnancy [3]. In such a case, serum IgG avidity tests can be performed to diagnose acute T. gondii infection in pregnant women with positive tests for both T. gondii IgG and IgM antibodies [3,4]. Avidity is described as the aggregate strength, by which a mixture of polyclonal IgG molecules reacts with multiple epitopes of the proteins. A low IgG avidity index confirms recent T. gondii infection, implying primary infection during pregnancy with a high risk of congenital T. gondii infection [5].

Our previous study of T. gondii antibody screening during pregnancy revealed an incidence of congenital T. gondii infection in Japan as 1.5% (7/469) among women with a positive test for T. gondii hemagglutination (HA)/IgG together with a positive or equivocal test for T. gondii IgM [6]. One hundred four (22.2%) women with low IgG avidity (<30%) measured by enzyme-linked immunosorbent assay (ELISA) were diagnosed with primary infection of T. gondii during the first trimester. All seven newborns with congenital T. gondii infection were born from mothers with low IgG avidity indices. An incidence of congenital T. gondii infection in the 104 women who had primary infection with low IgG avidity indices during pregnancy was found to be 6.7% (7/104). None of pregnant women with borderline or high IgG avidity indices gave birth to newborns with congenital T. gondii infection. Therefore, IgG avidity tests for women with a positive test for T. gondii HA/IgG together with a positive or equivocal test for T. gondii IgM were very useful to determine the risk for congenital T. gondii infection [6].

However, T. gondii IgG avidity tests have not been standardized or covered by health insurance in Japan. Recently, chemiluminescent microparticle immunoassay (CMIA) method for research use has been commercially available for T. gondii IgG avidity measurements in Japan, although its usefulness to detect maternal primary T. gondii infection during pregnancy or the risk for congenital T. gondii infection has been undetermined. The present study evaluated whether the CMIA compared with the ELISA was useful to detect primary T. gondii infection during pregnancy and to estimate the risk for congenital T. gondii infection in women with a positive test for T. gondii IgG together with a positive or equivocal test for T. gondii IgM.