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.

Help in the Choice of Automated or Semiautomated Immunoassays for Serological Diagnosis of Toxoplasmosis: Evaluation of Nine Immunoassays by the French National Reference Center for Toxoplasmosis

“…The Enzygnost assay only detected 16.7% of residual IgM antibodies. Given the presence of positive IgM and IgG antibodies in the first serum sample, a second-intention determination, using the same serum, of IgG avidity (when it is high) enabled confirmation of the infection as chronic (12,(40)(41)(42).…”

“…Ocular toxoplasmosis may be observed in immunocompetent patients (10). The diagnosis of toxoplasmosis is based mainly, but not only, on serological tests for IgG and IgM (11,12). These screening techniques enable the immunological status to be defined and the exposure risk for seronegative subjects to be estimated.…”

“…In clinical situations in which the date of the T. gondii infection is difficult to establish, Toxoplasma IgG avidity testing can be performed (14)(15)(16). Since 2006, the French National Reference Center for Toxoplasmosis (NRCT) has been entrusted with evaluation of the methods used for the serological diagnosis of toxoplasmosis (11,12) in accordance with national recommendations (17).…”

Identification, production and assessment of twoToxoplasma gondiirecombinant proteins for use in aToxoplasmaIgG avidity assay

“…However, cross-evaluation studies have shown variability in the performances of some well-known commercial assays. [31][32][33] Thus, efforts at improving IgG avidity assays are still needed.…”

Performance of the BioPlex 2200 Flow Immunoassay in Critical Cases of Serodiagnosis of Toxoplasmosis

“…The avidities of anti-Toxoplasma IgG antibodies were determined by using the Platelia Toxo IgG avidity test (Bio-Rad). Based on the manufacturer's instructions, the avidity indexes were classified as follows: Ͻ0.4, low avidity; 0.4 to Ͻ0.5, intermediate avidity; and Ն0.5, high avidity (high avidity suggests an infection Ͼ20 weeks previously) (13).…”