Studies on the Specificity of the Indirect Hemagglutination Test for Toxoplasmosis

Abstract

Evaluation of the indirect hemagglutination (IHA) test with the methylene blue dye (MBD) test for the diagnosis of toxoplasmosis on 1020 sera revealed that 19.3% of the sera were positive in the IHA test and negative in the MBD test. The basis for the reactivity in some of these sera was investigated by hemagglutination inhibition with a variety of specific and nonspecific antigens. Sera were absorbed with Toxoplasma gondii infected mouse peritoneal antigen, sheep red cells, normal mouse and rabbit serum, and mouse leukocytes. Evaluation of 63 IHA positive human sera, 36 negative and 27 positive by the MBD test, revealed the part that each of the non-Toxoplasma antigens played in producing false-positive reactions in the IHA test. In 52% of the 36 MBD negative-IHA positive sera and in 96% of the 27 MBD positive-IHA positive sera, reactivity was specific. With 70% of the sera with IHA titers of 1:200 or less the reactivity of the sera was absorbed with nonspecific antigen. In contrast, the reactivity in all sera with IHA titers greater than 1:200 was specific and could only by absorbed with Toxoplasma antigen. A one-step absorption procedure to eliminate false positive hemagglutination titers was developed and evaluated.