Interpretation of very low avidity indices acquired with the Liaison XL Toxo IgG avidity assay in dating toxoplasmosis infection

Congenital toxoplasmosis is an important cause of complications in pregnancy. Toxoplasmosis is often asymptomatic and thus serological tests are usually performed to screen for it. A first serum which exhibit both IgG and IgM may be due to nascent toxoplasmosis seroconversion, non-specific IgM reaction, or residual IgM. The IgG avidity test has been proposed to identify latent infections. A high index excludes recent toxoplasmosis whereas an intermediate or low index only suggests a recent infection, the caveats being that some people with latent Toxoplasma gondii infection show IgG with low or intermediate avidity. In this study, we investigated the ability of the Liaison XL Toxo IgG avidity (DiaSorin, Saluggia, Italy) assay to confirm recent infection when IgG avidity index is very low (≤ 0.1). Four thousand two hundred ninety-seven sera exhibiting both IgG and IgM were included and avidity was performed on the Liaison device according to the manufacturer's recommendations. One hundred twenty-six sera on the 297 sera which exhibited very low IgG avidity indices (≤ 0.1) could be exploited: 97% of sera with IgG avidity indices < 0.05 actually corresponded to recent infection (less than 3 months). A similar but less pronounced trend was observed for the sera exhibiting indices between 0.05 and 0.1 (69% corresponded to recent infections). The IgG avidity index data we obtained with the Liaison XL Toxo device are similar to those obtained with other devices. This body of consistent results underlines the interest of very low IgG avidity indices as a sign of probable recent toxoplasmosis.