Streptococcal Testing

"Pharyngitis" is a frequent office diagnosis, but the only common form of pharyngitis that requires antibiotic treatment is that caused by group A beta-hemolytic streptococci (GABHS). Approximately 15% to 30% of children and 5% to 10% of adults with sore throat have streptococcal pharyngitis. Suggestive clinical features include fever, no cough or rhinorrhea, tonsillar exudates or beefy-red pharynx, and tender anterior cervical lymphadenopathy.

For an acutely ill patient, the rapid antigen detection test (RADT) and traditional bacterial throat culture are available to identify GABHS. In the setting of pharyngitis and a typical syndrome, a positive culture or RADT is sufficient to begin treatment. The sensitivity of both is affected by throat swab technique. Using Dacron or rayon rather than cotton swabs will enhance the chances of detection. Testing should be done of both tonsils or tonsillar pillars and the posterior pharynx. Antigen or bacterial recovery from the throat is increased by rigorous swabbing. Avoid swabbing just the soft palate, tongue, buccal mucosa, or lips. The RADT has the advantage of giving immediate results, which allows for early antibiotic therapy and thus decreases the duration of illness, complications, and contagiousness. Current RADTs use immunoassay or nucleic acid testing to detect the group A streptococcal carbohydrate. This antigen disappears rapidly after antibiotic therapy, so a history of recent antibiotic use may give a false-negative result. The major limitation of rapid streptococcal tests is low sensitivity (75%-80% on average), but specificity is high (95%-98%). Therefore, a positive test can be accepted as evidence of disease and therapy begun without further testing. However, a negative result does not exclude the possibility of GABHS as the source of the pharyngitis. Children and adolescents with a negative RADT should have a throat culture for confirmation. Because of the low incidence of streptococcal pharyngitis and the extremely low risk of rheumatic fever in adults, the American Society of Infectious Diseases supports the use of RADT alone in adults, without confirmation by cultures (Bisno et al., 2002).

The throat culture is performed on sheep blood agar plate under aerobic conditions. If proper collection and plating technique are used, throat culture sensitivity is 95% to 96% and specificity 99.5%. With poor technique, sensitivity can be as low as 30%. To differentiate group A from other streptococci, a bacitracin disk is placed on this agar. Hemolysis is inhibited in over 95% of group A streptococci by bacitracin, whereas only 10% to 20% of groups C and G and a small percentage of group B are inhibited by bacitracin. Previous antibiotic use may diminish the colony count. If clinical conditions suggest the presence of other pharyngeal pathogens, such as Candida albicans, Corynebacterium diphtheriae, or Neisseria gonorrhoeae, the laboratory test should be altered because different collection and plating techniques are required. Throat culture results are generally reported 24 to 28 hours after plating. Antibiotic sensitivities are not routinely reported because GABHS is uniformly sensitive to penicillin.

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