[Fundamental study on serological diagnosis of amoebiasis. 3. Serological survey of amoebiasis in blood donors showing seropositive for syphilis].

Sera of 375 blood donors which were seropositive for syphilis were examined for antibodies against Entamoeba histolytica. Antibody prevalences against E. histolytica using complement fixation (CF) test were 12.0%, and enzyme-linked immunosorbent assay (ELISA) were 2.7%. The positive rates of antibodies in the CF test were significantly higher in sera showing positive by the Venereal Disease Research Laboratory (VDRL) test than in the VDRL-negative sera, and not related to the results of Treponema pallidum hemagglutination (TPHA) test. Only one sample in the VDRL-negative sera was positive by CF test. On the other hand, the positive rates or mean absorbance in ELISA were not correlated to the results of VDRL or TPHA test. The percent positivity in CF test became higher with the level of antibodies in VDRL test, but not that in ELISA. The level of antibodies in CF and VDRL test were weakly correlated. These results suggested that the results of CF test for E. histolytica antibodies were most likely false-positive in relation to the results of VDRL test. 10 (2.7%) of sera were positive by ELISA, but all ELISA-positive samples showed low ELISA titers and absorbance. 9 of the ELISA-positive samples showed indirect fluorescent antibody (IFA) titers of over 1:50, and the level of antibodies in the IFA test correlated to that in ELISA, so ELISA-positive persons seemed to have been previous infected or were asymptomatic cyst carriers.