Evaluation of the new Elecsys Toxo IgG avidity assay for toxoplasmosis and new insights into the interpretation of avidity results.

Detection and treatment of acute toxoplasmosis during pregnancy can avoid severe disease of the fetus. In this context, assessment of anti-Toxoplasma IgG avidity has been shown to exclude recent infection. The Elecsys Toxo IgG and IgM assays (Roche Diagnostics) have been validated for screening pregnant women and a new assay, Elecsys Toxo IgG Avidity, was recently developed. Our aims were to investigate the performance characteristics of this new avidity assay and explore whether additional information can be provided by avidity assays. The Elecsys assay was compared with the Vidas (bioMérieux) and Architect (Abbott) Avidity assays using two sets of serum samples (n = 291 and n = 255). The rate of general agreement between the Elecsys and Vidas assays was 74%, and that between the Elecsys and Architect assays was 83%. For 11% of the serum samples, avidity was high with the Vidas assay and within the gray zone with the Elecsys assay. None of the assays detected high-avidity antibodies in serum taken <4 months after infection. Avidity values of >90% were exclusively reported in sera taken >9 months after infection by the Elecsys and Architect assays. Almost all avidities of <19% with the Elecsys assay and <17% with the Architect assay corresponded to sera taken <3 and <2 months after infection, respectively. The Elecsys IgG Avidity assay can be used to exclude recent infection. New ways of interpreting the avidity result are also suggested: very high or low values could exclude infections within the last 9 months or help to confirm a recent infection, respectively. However, these potential interpretations require further investigation.