Anti-Toxoplasma IgG assays: What performances for what purpose? A systematic review

Chronic infection with Toxoplasma gondii is attested by the detection of specific anti-Toxoplasma IgG. A wide panel of serologic methods is currently marketed, and the most suitable method should be chosen according to the laboratory resources and the screened population. This systematic review of evaluation studies aimed at establishing an overview of the performances, i.e. sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of marketed anti-Toxoplasma IgG assays, and discussing their technical characteristics to guide further choice for routine diagnostic use. According to PRISMA guidelines, the search performed in PubMed and Web of Science databases recovered 826 studies, of which 17 were ultimately included. Twenty commercial anti-Toxoplasma IgG assays were evaluated, in comparison with an accepted reference method. Most of them were enzyme-immunoassays (EIAs, n = 12), followed by agglutination tests (n = 4), immunochromatographic tests (n = 3), and a Western-Blot assay (WB, n = 1). The mean sensitivity of IgG assays ranged from 89.7% to 100% for standard titers and from 13.4% to 99.2% for low IgG titers. A few studies pointed out the ability of some methods, especially WB to detect IgG early after primary infection. The specificity of IgG assays was generally high, ranging from 91.3% to 100%; and higher than 99% for most EIA assays. The PPV was not a discriminant indicator among methods, whereas significant disparities (87.5%–100%) were reported among NPVs, a key-parameter assessing the ability to definitively rule out a Toxoplasma infection in patients at-risk for opportunistic infections.