Bacterial Pharyngitis Workup

  • Author: Eric S Halsey, MD; Chief Editor: Burke A Cunha, MD   more...
 
Updated: Jan 11, 2012
 

Laboratory Studies

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.[26]

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 criterion 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.[27]

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,[14] 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  [28]

See the image below.

Rapid antigen detection test for group A beta-hemoRapid antigen detection test for group A beta-hemolytic streptococci.
  • 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.

Throat culture

  • 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.
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Imaging Studies

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 abscesses and sinusitis.

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Contributor Information and Disclosures
Author

Eric S Halsey, MD  Head, Virology Department, Naval Medical Research Center Detachment-Peru (NMRCD-Peru); Assistant Professor of Medicine, Uniformed Services University of the Health Sciences

Eric S Halsey, MD is a member of the following medical societies: Armed Forces Infectious Diseases Society, HIV Medicine Association of America, and Infectious Diseases Society of America

Disclosure: Nothing to disclose.

Specialty Editor Board

Kenneth C Earhart, MD  Deputy Head, Disease Surveillance Program, United States Naval Medical Research Unit #3

Kenneth C Earhart, MD is a member of the following medical societies: American College of Physicians, American Society of Tropical Medicine and Hygiene, Infectious Diseases Society of America, and Undersea and Hyperbaric Medical Society

Disclosure: Nothing to disclose.

Francisco Talavera, PharmD, PhD  Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Medscape Salary Employment

Gordon L Woods, MD  Consulting Staff, Department of Internal Medicine, University Medical Center

Gordon L Woods, MD is a member of the following medical societies: Society of General Internal Medicine

Disclosure: Nothing to disclose.

Eleftherios Mylonakis, MD  Clinical and Research Fellow, Department of Internal Medicine, Division of Infectious Diseases, Massachusetts General Hospital

Eleftherios Mylonakis, MD is a member of the following medical societies: American Association for the Advancement of Science, American College of Physicians, American Society for Microbiology, and Infectious Diseases Society of America

Disclosure: Nothing to disclose.

Chief Editor

Burke A Cunha, MD  Professor of Medicine, State University of New York School of Medicine at Stony Brook; Chief, Infectious Disease Division, Winthrop-University Hospital

Burke A Cunha, MD is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, and Infectious Diseases Society of America

Disclosure: Nothing to disclose.

Additional Contributors

Thank you to Donald Minnich and Dennis Clark for the preparation and photography of the microbiology specimens.

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Picture of Streptococcus pyogenes at 100 X magnification.
Rapid antigen detection test for group A beta-hemolytic streptococci.
Posterior pharynx with petechiae and exudates in a 12-year-old girl. Both the rapid antigen detection test and throat culture were positive for group A beta-hemolytic Streptococcus.
 
 
 
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