Evaluation of commercial latex agglutination reagents for grouping streptococci

A total of 155 strains of beta-hemolytic streptococci were serologically grouped by conventional techniques (Lancefield extraction and capillary precipitin testing) and by latex agglutination (LA). Agreement between conventional and LA techniques was 97% when the instructions of the manufacturer for the LA technique were followed. Agreement of 99% was obtained when modified autoclave extracts were used as antigens in the LA procedure. A total of 82 strains of non-beta-hemolytic streptococci were also tested by conventional, prescribed LA, and modified autoclave procedures. The agreement between conventional techniques and both LA procedures was 76%. However, when serological cross-reactions in the conventional grouping procedures were considered as errors, the accuracy of identification of both LA procedures was 88% among the non-beta-hemolytic strains. Of 13 strains of Streptococcus bovis, 10 did not react with the LA group D reagent but were serogroup D by conventional techniques. More S. bovis strains were grouped by the LA technique when extracts of 20 ml of broth cultures were used as antigens; however, cross-reactions were observed with non-group D strains when this technique was applied to them.