Improvement of the indirect hemagglutination test for the detection of antibodies to Streptococcus pyogenes

RESUMO

An indirect hemagglutination test for a seroepidemiological survery of Streptococcus pyogenes infection was standardized. This is an improved modification of the indirect hemagglutination test which utilizes an unstable reagent prepared with fresh blood cells. Two types of bacterial antigens represented by extracellular products and purified streptolysin O were assayed but only the former antigen gave good results. Pretreatment of the bacterial antigen with 0.15 M NaOH and neutralization to pH 5.5, as well as postfixation of sensitized red cells with 0.1 per cent glutaraldehyde at 56 degrees Celsius for 30 min were found to be essential to give long stability to the reagent in liquid suspension, at least 9 months at 4 degrees Celsius. A total of 564 serum samples with high, moderate and low anti-streptolysin 0 antibodies as determined by the neutralization assay were studied by the indirect hemagglutination test using the new reagent. The sensitivity, specificity, efficiency, positive predictive value and negative predictive value of the test in relation to the neutralization assay were 0.950, 0.975, 0.963, 0.973, and 0.955, respectively. The kappa agreement index between the two techniques was high (0.926) and ranked as "almost perfect". Antibody levels detected by both techniques also presented a high positive correlation (r(s)=0.726). Five reagent batches sucessively produced proved to be reproducible. Thus, the improved indirect hemagglutination test seems to be useful for public health laboratories.