Comparison of four commercially available avidity tests for Toxoplasma gondii-specific IgG antibodies

Abstract

Toxoplasma infection in pregnant women may cause congenital toxoplasmosis. Diagnosis of infection is based on serological tests aimed at detecting IgM and IgG antibodies against Toxoplasma gondii. However, IgM antibodies are not an accurate marker for discriminating between acute and latent infection. Detection of residual or persistent IgM may occur months or even years after primary infection, while the IgG avidity test is a rapid means of identifying latent infections in pregnant women who exhibit both IgG and IgM anti-Toxoplasma antibodies on initial testing during pregnancy. In this study, we assessed and compared the performances of four commercially available Toxoplasma IgG avidity tests in immunocompetent and immunocompromised patients with acute and latent toxoplasmosis. The positive predictive value of high avidity to confirm latent toxoplasmosis was 100% for all the assays, indicating that high avidity is a hallmark of latent infection. However, the negative predictive value of high avidity ranged from 99.2% (bioMérieux) to 95.3% (Abbott), indicating that acute toxoplasmosis could not be reliably diagnosed based on low IgG avidity alone. Thus, the avidity test provides a rapid means for identifying latent Toxoplasma infection in immunocompetent pregnant women presenting both IgG and IgM anti-Toxoplasma antibodies on initial testing. In terms of cost-effectiveness, avidity testing is a powerful tool that optimizes screening and follow-up of pregnant women while minimizing the costs of screening by avoiding subsequent costly maternal and fetal investigation and unnecessary treatment. The cheapest assay, Vidas Toxo IgG Avidity, also had the best performance for the diagnosis of latent toxoplasmosis.

Fig 1

Mean kinetics of IgG levels (A) and IgG avidity values (B) with four immunoassays in 67 sera of 56 pregnant women with acute toxoplasmosis.

Fig 2

Kinetics of IgG avidity for the four immunoassays in 67 samples taken from 56 pregnant women with acute toxoplasmosis and confirmed seroconversion. All the women were treated for acute toxoplasmosis after the first serum was tested. The horizontal lines represent the upper and lower cutoffs of the gray zone for each assay. Each symbol represents a single patient.

Fig 3

IgG avidity results with the four immunoassays in IgG- and IgM-positive sera in latent toxoplasmosis (n = 55). The black squares represent treated or immunocompromised patients; the circles represent the immunocompetent patiens. The horizontal lines represent the upper and lower cutoffs of the gray zone for each assay.

Fig 4

IgG avidity results with the four immunoassays in IgG-positive and IgM-negative sera in the latent toxoplasmosis population (n = 84). The black squares represent treated or immunocompromised patients; circles represent immunocompetent patients. The horizontal lines represent the upper and lower cutoffs of the gray zone for each assay.