Antibodies against the group polysaccharide of group A streptococci were estimated by means of a haemagglutination reaction. In this reaction human erythrocytes of blood group O were sensitized with polysaccharide esterified with myristoylchloride. The optimal conditions of the reactions were determined by varying the ester group content in the antigen and the amount of ester used for sensitization. The specificity of the reaction could be established by reacting sensitized erythrocytes with homologous and heterologous sera and by absorption experiments. Antistreptococcal group A polysaccharide titres (ASPAT) and antibody levels to streptolysine O and DNase-B were compared in a group of 52 children with proved streptococcal infection and in 52 age- and season-matched controls. Antibody levels were significantly higher in the patient group than in the controls. In the ASPAT there was clearly less overlap between patients and controls than in both other reactions. In the patient group the ASO titres were raised above normal in 27 cases (51·9%), anti-DNase-B titres in 18 (34·6%), and ASPAT in 40 (76·9%). Taken together the three reactions gave a positive score in 51 cases (98·1%) in the patient group against 17 cases (32·7%) in the controls. A positive antibody response is usually defined as a rise of two dilution increments between the acute and convalescent sera. According to this definition the ASPAT showed a response in 42%, ASO and/or DNase-B in 42%, and the three reactions taken together in 68% of paired sera from patients. It is believed the ASPAT will prove a welcome addition to the diagnostic outfit when the presence of streptococcal infection in children is considered.
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