Simplified coagglutination test for serological grouping of beta-hemolytic streptococci

A new coagglutination kit consisting of a plastic slide with dried antibody-coated staphylococci was evaluated for the grouping of beta-hemolytic streptococci of groups A, B, C, and G. The test was compared with the classical precipitation test, using hot formamide antigen extracts with 224 strains of groups A, B, C, and G streptococci. An agreement of 100% was found between the new coagglutination and the classical precipitation procedures. No false-positive results were obtained with group F and D streptococci; however, group L streptococci reacted with the group A reagent. The test procedure could be shortened by using suspensions of colonies from overnight cultures on blood agar plates in small volumes of Todd-Hewitt broth without further incubation. All 74 strains of groups A, B, C, and G were correctly identified from suspensions in Todd-Hewitt broth; however, suspending the colonies in 0.9% saline or phosphate-buffered saline resulted in lower sensitivities.