Evidence-based diagnosis of toxoplasma infection

The aim of this study was to compare the performance of one in-house and four commercially available toxoplasma assays with the Sabin-Feldman dye test. One hundred fifty-seven routine sera and 20 potentially cross-reactive sera were tested blindly using four commercial assays: Abbott AxSym IgG (Abbott Laboratories, UK), Captia Select Toxo-G (Trinity Biotech, UK), Toxreagent 'Eiken' (Eiken Chemical, Japan) and Toxolatex Fumouze (Fumouze Laboratoires, France); an in-house IgG and IgM enzyme immunoassay (EIA); and the gold standard Sabin-Feldman dye test. The sensitivity, specificity and the values using the formulae for numbers needed to diagnose (NND) and the cost per positive diagnosis (CPPD) were calculated for each assay. These formulae use the sensitivity and specificity of the assay to allow for evidence-based comparisons between assays. The NND values for the in-house IgG EIA, AxSym, Eiken and Fumouze latex kits were similar (1.21-1.24), whereas the Captia yielded the poorest value (1.33). The in-house EIA IgG had the lowest CPPD value (pound sterling 0.57/$0.91), and the Fumouze and Eiken latex kits had the lowest CPPD values for commercial assays (pound sterling 1.42/$2.27 and pound sterling 1.81/$2.90, respectively). Both assays were simple and straightforward to use. Specialist laboratories should opt to use in-house assays, as they were most cost-effective. Although nonspecialist laboratories could use commercial assays, specimens from immunocompromised patients and patients with ocular disease should be forwarded to specialist laboratories without prior testing.