Selective streptococcal agar versus blood agar for detection of group A beta-hemolytic streptococci in patients with acute pharyngitis.

In a study on acute pharyngitis in general practice, we compared a selective group A streptococcal agar (ssA) for the recovery of group A beta-hemolytic streptococci (GABHS) with sheep blood agar. All plates were incubated at 36 degrees C in an atmosphere reinforced with 5% CO2 for 48 h with a first reading after 24 h. A total of 197 GABHS isolates were obtained from 721 throat cultures on both media. The recovery of GABHS was significantly higher after 48 h of incubation for both media. With the ssA plate, we detected significantly more GABHS after 24 h as well as after 48 h of incubation. The ssA plate reduced normal flora qualitatively and quantitatively. In conclusion, ssA is more sensitive and specific for the detection of GABHS than sheep blood agar and moreover easier to read. We recommend incubation for 48 h.