Testing for Group A Streptococcus
Diagnostic methods for Group A Strep include culture, rapid antigen testing and
molecular. All diagnostic methods begin with throat swab collection. The optimal
swab specimen comes from the tonsils and posterior pharyngeal wall; poor specimen
collection can lead to a false negative result.
Group A Strep grows readily on routine media, but it can be isolated more easily
using selective media that inhibit the growth of normal pharyngeal flora. Most laboratories
inoculate throat swabs on 5% sheep blood agar containing trimethoprim-sulfamethoxazole.
A bacitracin disk that contains 0.04U of bacitracin is also placed at the initial
inoculation of the swab. After overnight incubation at a temperature of 35-37°C,
beta-hemolytic colonies that grow despite inhibition of the antibiotic disk are
presumed to be composed of Group A Strep. Cultures that are negative for Group A
Strep after 24 hours are held for another overnight incubation and reexamined.2
Culture holds several drawbacks. It has a 48 hour turnaround time to confirm a negative
result. Furthermore, prior antibiotic treatment of the patient may give a false
negative result. Culture sensitivity can be as low as 71%.5
Rapid Antigen Detection Test (RADT)
Most rapid antigen tests use antibodies for the detection of the group A carbohydrate
antigen. The indicator systems used are latex agglutination or enzyme immunoassay.
Rapid antigen testing sensitivity and specificity range from 61%–80% and 98%-99.5%
Molecular testing for Group A Strep has become commercially available, but the sensitivity
and specificity of these assays vary. Polymerase chain reaction (PCR) may be used
for primary testing of Group A Strep or as a backup to rapid antigen testing instead
of a plated culture.6 PCR requires special equipment that must be purchased
to perform extractions and/or amplifications.