Value of the IgG avidity in the diagnosis of recent toxoplasmosis: A comparative study of four commercially available anti-Toxoplasma gondii IgG avidity assays

The determination of the anti-Toxoplasma immunoglobulin G (IgG) avidity is a diagnostic method which has been intensively used for differentiating of recent and more distant Toxoplasma gondii infection in a single serum sample. Finding of low avidity anti-Toxoplasma IgG antibodies usually indicates recently acquired infection, while the presence of high avidity IgG antibodies contradicts to recently acquired infection. Recently, a number of commercial kits have been introduced for determining the avidity of anti-Toxoplasma IgG antibodies. Therefore, it seemed advisable to make comparative studies in order to investigate the correspondences and discrepancies in the results obtained by the different kits. We compared and analysed the results obtained with four commercially available anti-Toxoplasma IgG avidity assays. They were expected to give well correlated results, but some differences between the kits were observed in the ranking of anti-Toxoplasma IgG avidity ("high" or "low" or "borderline"). Among others, differences in the dilution method or differences in the type, quality or quantity of the Toxoplasma antigen used in the different assays might explain the observed divergences.