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Testing

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.

Culture

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% respectively.5

Molecular

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.

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