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June 1984

Primary Plate Identification of Group A Streptococcus on a Selective Medium: Efficiency in an Office Practice

Author Affiliations

From the C. Henry Kempe Center for Investigative Pediatrics, and the Departments of Pathology and Pediatrics, The Children's Hospital of Denver, and Pediatrics and Microbiology/Immunology, University of Colorado School of Medicine, Denver.

Am J Dis Child. 1984;138(6):589-591. doi:10.1001/archpedi.1984.02140440073019

• 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. Bacitracinresistant β-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 β-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 β-hemolytic colonies are thereafter immunologically tested for group A Streptococcus antigen.

(AJDC 1984;138:589-591)

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