- Last Update On : 2013-01-13
Group A beta-hemolytic streptococcus (GAS, S. pyogenes) is responsible for up to 30% of cases of acute pharyngitis. The clinical features of pharyngitis caused by group A strep include purulent exudate, fever, and anterior cervical adenopathy. Beta-hemolytic streptococci groups C & G can cause pharyngitis with symptoms and signs that are indisitinguishable from group A strep infections, but are more commonly found in adolescents and young adults.
The mainstay of diagnosis for group A strep pharyngitis is rapid antigen testing of throat swabs. The obvious advantage of rapid antigen testing is the immediate availability of results and the ability to perform testing in the clinic setting. However, a major disadvantage of these tests is less than optimal sensitivity, so that culture back-up of negative tests is essential. Culture is also necessary to identify the presence of group C or G streptococci, which are not detected by rapid antigen tests. Unfortunately, final culture results are generally not available until 24-48 hours after specimen collection.
The American Academy of Pediatrics recommends that all negative rapid antigen assays in children be followed by culture confirmation. This additional step is less important in adults due to the lower incidence of GAS pharyngitis and decreased risk of serious complications such as acute rheumatic fever and acute glomerulonephritis.
When obtaining samples for both rapid antigen assays and culture, two pharyngeal swabs should be obtained: one for each test. If only one swab is submitted, the culture plates should be inoculated first.
Several approaches can be used to identify GAS in throat cultures. The initial isolation media is sheep blood agar and/or a selective streptococcal media that contains trimethoprim-sulfamethoxazole. After 24 to 48 hours incubation, S. pyogenes colonies on blood agar are 1-2 mm in diameter, grayish white, transparent, circular, convex, mat or glossy and surrounded by a zone of hemolysis. These colonies can be further identified by performing a catalase test and gram stain. Beta-hemolytic, catalase negative, gram positive cocci in pairs or chains can be further confirmed as GAS by using the PYR, latex agglutination or DNA probe tests. These tests allow differentiation of GAS from oral flora and other causes of bacterial pharyngitis.