A study of variation in the indirect hemagglutination antibody test for toxoplasmosis

The indirect hemagglutination test (IHA) was compared both qualitatively and quantitatively to that of the indirect fluorescent antibody (IFA) technique for the detection of antibodies to Toxoplasma gondii. The coefficient of correlation between the two methods was +0.9561. Of the 89 samples tested, no significant disagreement between the IFA titers and mean titers of the IHA test was detected. Quantitative evaluation revealed 93.3 percent of the sera positive in both methods were within a one-tube dilution or same dilution, and 100 percent were within a two-tube (four-fold) dilution. A statistical evaluation of internal reproducibility of titer in the IHA test, utilizing 30 paired samples, indicated that titers by this method can be reproduced on a within-day basis within limits of +/- 1 two-fold dilution with 95 percent confidence. Day-to-day reproducibility of titers were 100 percent within limits of +/- 1 four-fold dilution. Because of its high reproducibility, prolonged stability, and ability to produce standardized results, the IHA technique is recommended over the IFA for mass screening.