Primary plate identification of group A Streptococcus on a selective medium. Efficiency in an office practice

The efficiency of group A Streptococcus detection and identification in an office practice was studied using a selective blood agar plate containing sulfamethoxazole and trimethoprim and primary plate bacitracin disk speciation. All results were confirmed by conventional bacteriologic and immunologic techniques the next day by a reference laboratory. In all, 1,591 cultures were processed, of which 156 (10%) could be confirmed the next morning to have group A Streptococcus by primary disk susceptibility interpretation. Bacitracin-resistant beta-hemolytic colonies, which could be immunologically confirmed as group A Streptococcus grew from only three cultures (0.1%). Fifty-six (3%) of the cultures had too few beta-hemolytic colonies to determine bacitracin susceptibility, of which 47 were later proved to be group A Streptococcus. On all preliminary negative cultures, 1% demonstrated group A Streptococcus after incubation for an additional 24 hours. If a selective blood agar plate with primary bacitracin disk susceptibility speciation is used in an office laboratory setting, 99% of all cultures can be accurately interpreted within 24 hours of incubation, providing that those plates with limited growth of beta-hemolytic colonies are thereafter immunologically tested for group A Streptococcus antigen.