A new soluble antigen preparation of Toxoplasma gondii and its use in serological diagnosis.

A new method of solubilizing Toxoplasma gondii in 5% saponin and 5% octyl glucoside in combination (S-Ag) is described. When analysed by IEP, seven antigens could be detected. Four of these antigens (Ag 1, 2, 3 and 12) were shown by 125I-iodination to be of membrane origin. The remaining three (Ag 4, 5 and 6) were shown to be of intracellular origin and comprise the antigenic profile of more conventional preparations used in serodiagnosis. The S-Ag preparation was tested by ELISA (S-ELISA) in parallel with a freeze-thaw preparation containing only Ag 4, 5 and 6 (FT-ELISA), against the IgG-IFAT. The regression of the FT-ELISA against the IgG-IFAT was poor and there was a high incidence (28.9%) of false negative results. A second order polynomial provided a better fit and this predicted FT negative results on sera with IgG-IFAT titres lower than 1:1024. From this titre upwards, the test was specific but insensitive. The S-ELISA was more specific (5.6% false negatives) and more sensitive; polynomial terms could not provide a better fit. The improvement in the test using the S-Ag is considered to be due to the additional membrane antigens. The possible role of this solubilized antigen in future serodiagnosis is discussed.