Suitability of a throat culture method for evaluation of group A streptococcal antigen detection kits

Previous reports have indicated a wide variation in observed sensitivity of antigen-detection kits for group A streptococci. Before undertaking an evaluation of these new kits, the sensitivity of the throat culture technic routinely used by this laboratory was reexamined. Each throat swab was directly inoculated to sheep blood agar containing trimethoprim-sulfamethoxazole (SXT X BA) and drug-free sheep blood agar (SBA) plates. Swabs were then washed in saline and the saline used to inoculate one more of each type of medium. SXT X BA cultures were incubated aerobically (5 to 10% CO2), and SBA cultures were incubated anaerobically, both for two days at 35 degrees C. From 726 patients, 164 (22.6%) of the specimens contained group A streptococci, 99% detected on directly inoculated cultures and 100% on cultures inoculated with the saline wash. Either an aerobically (CO2) incubated SXT X BA or an anaerobically incubated SBA, directly inoculated and held for two days, appears to offer a satisfactory reference culture method for the recovery of group A streptococci.