Toxoplasma gondii-specific IgG avidity testing in pregnant women.

Abstract Background; The parasite Toxoplasma gondii (Tg) can cause congenital toxoplasmosis following primary infection in a pregnant woman. It is therefore important to distinguish between recent and past infection when both Tg-specific IgM and IgG are detected in a single serum in pregnant women. Tg-specific IgG avidity testing is an essential tool to help to date the infection. However, interpretation of its results can be complex. Objectives To review Tg-specific avidity testing benefits and limitations in pregnant women, in order to help practitioners to interpret the results and adapt the patient management. Sources PubMed search with the keywords avidity, toxoplasmosis and Toxoplasma gondii for articles published from 1989 to 2019. Content Tg-specific IgG avidity testing remains a key tool for dating a Tg infection in immunocompetent pregnant women. Several commercial assays are available and display comparable performances. A high avidity result obtained on a first-trimester serum sample is indicative of a past infection, which occurred before pregnancy. To date, a low avidity result must still be considered as non-informative to date the infection, although some authors suggested that very low avidity results are highly suggestive of recent infections depending on the assay. Interpretation of low or grey zone avidity results on a first-trimester serum sample, as well as any avidity result on a second or third-trimester serum sample, is more complex and requires recourse to expert toxoplasmosis laboratories. Implications Although used for about 30 years, Tg-specific avidity testing has scarcely evolved. The same difficulties in interpretation have persisted over the years. Some authors proposed additional thresholds to exclude an infection of less than 9 months, or in contrast to confirm a recent infection. Such thresholds would be of great interest to adapt management of pregnant women and avoid unnecessary treatment: however, they need confirmation and further studies.