Specificity study of kits for detection of group A streptococci directly from throat swabs

A total of 78 Streptococcus strains, 15 Staphylococcus strains, and 2 Stomatococcus strains were used to test the specificity of 18 different antigen detection systems for group A streptococci and five products that detect a specific enzyme associated with group A streptococci. All streptococcal strains possessing the group A antigen were correctly identified with 31 different lots of reagents in the 18 antigen detection systems. The specificities of the 31 different lots of reagents ranged between 88.5 and 100%. A limited number of nonspecific reactions were observed with Enterococcus gallinarium, group C Streptococcus strain C23, and Staphylococcus aureus F49 and Cowan 1. The antigen detection kits that used enzymes as the extraction reagent gave slightly more specific results than did the kits that used chemical extraction reagents. The reagents in the five kits designed to detect the enzyme pyroglutamic acid arylamidase in Streptococcus pyogenes reacted positively with S. pyogenes (group A streptococcus); however, the reagents also reacted positively with all group D enterococcal streptococci and with about half of the staphylococcal strains treated. The nonspecificity of tests based on pyroglutamic acid arylamidase detection would seem to limit the usefulness of these kits with mixed cultures.