Laboratory Tests
The diagnosis of GABHS tonsillitis can be confirmed by culture of samples obtained by swabbing both tonsillar surfaces and the posterior pharyngeal wall and plating them on sheep blood agar medium. The isolation rate can be increased by incubating the cultures under anaerobic conditions and using selective media. A single throat culture has a sensitivity of 90%-95% for the detection of GABHS. A small percentage of false-negative results are part of the characteristics of the tests used but are also possible if the patient has received antibiotics prior to testing. Identification requires 24 to 48 hours by culture but rapid screening tests (10–60 minutes), which have a sensitivity of 85-90%, are available. Older antigen tests detect the surface Lancefield group A carbohydrate. Newer tests identify GABHS serotypes using nucleic acid (DNA) probes or polymerase chain reaction. Bacterial culture should be performed in cases of a negative rapid streptococcal test.
True infection with GABHS, rather than colonization, is defined arbitrarily as the presence of >10 colonies of GABHS per blood agar plate. However, this method is difficult to implement because of the overlap between carriers and infected patients. An increase in antistreptolysin O (ASO) streptococcal antibody titer 3–6 weeks following the acute infection can provide retrospective evidence of GABHS infection and is considered definitive proof of GABHS infection.
When GAHBS is not isolated, the clinician should seek other potential pathogens. However, many of these organisms are part of the normal flora residing in the pharynx, making interpretation of the results difficult. A finding of a membrane in the throat should prompt a search for corynebacteria. Culture should be obtained from beneath the membrane, and use of a special moisture-reducing transport medium is necessary. The material may be inoculated on a Loeffler slant, tellurite plate, or blood agar plate. Identification by fluorescent antibody technique is also possible. Viral cultures are available, as well as rapid tests for some viruses (e.g., respiratory syncytial viruses). A heterophile slide test or other rapid test for infectious mononucleosis can provide a specific diagnosis.
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