Suitability of a Throat Culture Method for Evaluation of Group A Streptococcal Antigen Detection Kits

Abstract

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.

Suitability of throat culture procedures for detection of group A streptococci and as reference standards for evaluation of streptococcal antigen detection kits

“…Microbiol. 1989, C123, p. 414), 9 to 35% in two studies with C02-supplemented aerobic incubation (31,36), and 2 to 31% in seven studies using anaerobic incubation (17,22,29,31,35,36; Kellogg and Bankert, Abstr. Annu.…”

“…A second day of incubation of SBA containing sulfamethoxazole and trimethoprim (SBA-SXT) resulted in increased recoveries of the organism of 62 to 63% in two studies with aerobic incubation (31, 35), 8 to 52% in three studies with C02-supplemented aerobic incubation (22,27,31), and 0 to 38% in three studies using anaerobic incubation (27,31,35). Of 23 medium-atmosphere combinations (from nine studies; Table 1) for which recoveries at 1 and 2 days of incubation were compared, recovery in 22 (96%) was improved on the second day and the increase was more than 5% in 20 (87%). To achieve maximum detection of group A streptococci, a high PVN, and a high degree of confidence in results from studies which include streptococcal cultures, those cultures negative at 1 day should be incubated for a second day, regardless of medium or atmosphere selected, unless a thorough in-house investigation indicates that a second day of incubation has been documented to be unproductive.…”

“…Only one of the eight studies found that SBA (incubated anaerobically) provided significantly greater detection of S. pyogenes than did SBA-SXT, but the cultures were held for just 18 to 24 h (11). Variations in reports of the recovery of group A streptococci on SBA-SXT compared with recovery on noninhibitory SBA are most likely due to factors that include variations in the thymidine content ofthe different media used (8,11,27), the different susceptibilities of some normal floral species to SXT (8), variations among different populations in the species making up the normal flora (35), and significantly improved recovery of S. pyogenes when SBA-SXT cultures are incubated for 48 h (22,27,31,35). Ail four investigations comparing noninhibitory SBA with SBA-SXT and in which cultures were incubated for 48 h found that recovery of group A streptococci on SBA-SXT was either equivalent or superior to that on the best of the SBA-atmosphere combinations (22,27,31,35).…”

“…Variations in reports of the recovery of group A streptococci on SBA-SXT compared with recovery on noninhibitory SBA are most likely due to factors that include variations in the thymidine content ofthe different media used (8,11,27), the different susceptibilities of some normal floral species to SXT (8), variations among different populations in the species making up the normal flora (35), and significantly improved recovery of S. pyogenes when SBA-SXT cultures are incubated for 48 h (22,27,31,35). Ail four investigations comparing noninhibitory SBA with SBA-SXT and in which cultures were incubated for 48 h found that recovery of group A streptococci on SBA-SXT was either equivalent or superior to that on the best of the SBA-atmosphere combinations (22,27,31,35). In addition, SBA-SXT cultures incubated either aerobically (5 to 10% C02) or anaerobically were shown to result in detection rates of 96.9% (22) to 98.0% (27) and 96.0% (35) to 100% (27), respectively, of the total group A streptococcal isolates recovered, while anaerobically incubated SBA cultures led to recovery of 84.6 to 98.8% of the total isolates (22,27,35).…”

“…Ail four investigations comparing noninhibitory SBA with SBA-SXT and in which cultures were incubated for 48 h found that recovery of group A streptococci on SBA-SXT was either equivalent or superior to that on the best of the SBA-atmosphere combinations (22,27,31,35). In addition, SBA-SXT cultures incubated either aerobically (5 to 10% C02) or anaerobically were shown to result in detection rates of 96.9% (22) to 98.0% (27) and 96.0% (35) to 100% (27), respectively, of the total group A streptococcal isolates recovered, while anaerobically incubated SBA cultures led to recovery of 84.6 to 98.8% of the total isolates (22,27,35).…”