Is Sabin-Feldman dye test using T. gondii tachyzoites from animal inoculation still the best method for detecting Toxoplasma gondii antibodies?

Although the Sabin-Feldman dye test is the gold standard for detecting Toxoplasma antibodies in human, it is performed only in reference laboratories because live virulent T. gondii are used for the test. We collected 210 human serum samples and tested them by the dye test using in vivo tachyzoites (conventional method) then compared these results with three other methods: a dye test using cell culture-derived T. gondii tachyzoites and indirect immunofluorescent antibody tests (IFAT) using in vivo and in vitro tachyzoites. We found the conventional dye test detected the highest percent of cases (4.3%), followed by the IFAT using parasites from mice (3.8%), then the dye test and the IFAT using cell culture tachyzoites (both 2.8%). Agreement with the dye test when using mouse and cell culture derived tachyzoites was 96.7%. Using in vivo tachyzoites for the dye test and the IFAT gave 94.3% agreement, while using in vitro tachyzoites gave 94.8% agreement. When compared with the conventional dye test, the IFAT had 75% sensitivity and 100% specificity. The T. gondii tachyzoites obtained from cell culture had a lower virulence, as indicated by a three times longer survival period in the inoculated mice. We favor the conventional dye test as the gold standard for Toxoplasma antibody detection. In vitro tachyzoites can be used routinely in the dye test but false negative results may occur in some cases. The IFAT, using either in vivo or in vitro tachyzoites, are alternatives for laboratories where provision of live tachyzoites is limited.