Bacterial Pharyngitis Workup
- Author: Maria A Carrillo-Marquez, MD; Chief Editor: Michael Stuart Bronze, MD more...
The clinical features of GABHS pharyngitis overlap significantly with that caused by non-GABHS. Microbiological testing provides data to help determine who may benefit from GABHS-directed therapy. Laboratory evaluation of pharyngitis falls into two broad categories: rapid antigen detection tests (RADT) and throat culture. A throat culture is demonstrated in the video below.
RADTs offer the advantage of a speedy diagnosis, allowing for the proper administration, as well as proper withholding, of antibiotics. Drawbacks of RADTs include a higher cost and lower sensitivity compared with culture. While throat culture remains the gold standard for diagnosis of GABHS pharyngitis, it has a 24-48 hour turnaround time and entails more technical involvement. Both RADTs and throat culture cannot be used to differentiate between infection and colonization and, in some cases, may influence a physician to overuse antibiotics. For example, a child with coronavirus pharyngitis and GABHS colonization may be prescribed antibiotics based on a false–positive RADT result. No matter what type of test is used in the outpatient setting, judicious selection of patients to be screened is imperative in order to avoid a large number of false-positive results.
Samples for RADT or throat culture should be obtained from the posterior pharynx or tonsils. Samples from the oral cavity result in a greatly reduced sensitivity.
Test of cure is not usually indicated except in special situations, including in patients with a history of rheumatic fever, in those who live in a community with an increased incidence of rheumatic fever or poststreptococcal glomerulonephritis, and in patients involved in outbreaks of GABHS pharyngitis in closed or semiclosed communities. Consideration should also be given to members of a family in whom "ping-pong" spread is presumed.
Antistreptococcal antibody tests have no role in the diagnosis of acute bacterial pharyngitis. However, they may be used to confirm a history of exposure to GABHS in patients with suspected poststreptococcal glomerulonephritis or acute rheumatic fever.
Rapid antigen detection tests 
See the image below.
All RADTs yield high specificity, allowing for prompt treatment of GABHS pharyngitis without the concern of false-positive results.
Initial RADTs relied on latex agglutination to identify cell wall carbohydrates obtained after acid extraction, a method associated with low sensitivity.
Newer RADTs use optical immunoassay (OIA) technology to identify cell wall carbohydrates. These yield a sensitivity that may be similar to that of throat culture. Nevertheless, before removing confirmatory throat cultures from any given clinical practice, verification of increased sensitivity is recommended.
A newer generation of rapid tests uses nucleic acid identification to identify GABHS-specific sequences. Such assays yield a specificity of 95%-100% and sensitivity in the range of 86%-95%. Although these tests provide an answer in hours, they rely on equipment not available in most outpatient settings and often need to be performed at a location other than the office.
Considered the criterion standard of GABHS pharyngitis diagnosis, throat culture involves obtaining a sample from the posterior pharynx and tonsils and plating on sheep blood agar.
Bacitracin disks aid in differentiation of GABHS from other beta-hemolytic streptococci. A large zone of inhibition is found around GABHS but not around non–beta-hemolytic streptococci.
Cell wall carbohydrate detection assays, applied directly to the cultured bacteria, may also differentiate GABHS from other streptococci.
Imaging studies have no role in the diagnosis of bacterial pharyngitis. Lateral neck films may help to confirm the diagnosis of acute epiglottitis. CT scanning may aid in the diagnosis of some of the suppurative complications of pharyngitis, including retropharyngeal or deep neck abscesses, lymphadenitis, and sinusitis.
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