Commercial kits are not equally sensitive in detecting the group antigen of group A streptococcus

The relative sensitivities of four commercially available kits for the demonstration of the group antigen of group A streptococcus were estimated in laboratory experiments. Two kits gave positive reactions with swabs charged with approximately 10(5) colony-forming units of group A hemolytic streptococci of five common T-types. The two other kits required inocula ten times higher. Application of the antigen detection reagents from each kit to antigen extracts prepared by extraction reagents from the other kits revealed that the differences in sensitivity were largely attributable to differences in the extraction abilities. The four kits did not differ appreciably in their ability to demonstrate group A antigen in human pus mixed with group A streptococci; the minimum inoculum detectable was approximately 10(6) colony-forming units per 0.04 ml of pus.