Laboratory Studies

GABHS rapid antigen detection test

This is the preferred method for diagnosing GAS infection in the emergency department because of difficulties with culture follow-up. (See the image below.)

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

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Only patients with a high clinical likelihood of GAS pharyngitis should be tested. Patients with a Centor score of 0-1 should be treated symptomatically without testing.^[22] (On the other hand, in a multicenter, prospective, cross-sectional study, Cohen et al suggested that all children with pharyngitis undergo the rapid antigen detection test for GAS, stating that assessment of signs and symptoms is an inefficient means of determining in which patients the test should be used.^[23])

Antigens are specific, but sensitivities vary. The sensitivity of the GABHS rapid antigen detection test is 70-90%, and the specificity is 90-100%, depending on the manufacturer.^{[24, 25, 26, 27]}Children with a negative antigen test should have a follow-up culture unless the antigen being used in the office has been shown to be as sensitive as a culture.^[17]

The use of a GABHS rapid antigen detection test can decrease the use of unnecessary antibiotics in pediatric patients when used properly.^[28]

Adults do not need follow-up culture after a negative antigen test because of the low incidence of GAS in this population.^[13]

Throat culture

This is the criterion standard for diagnosis of GAS infection (90-99% sensitive).^[29]Although less expensive than the rapid antigen detection test, it is not be the best test to use in the emergency department because of difficulty with follow-up. The guidelines that recommend cultures for GAS screening are aimed at office-based practices and not the emergency department.

Patients can be treated up to 9 days after onset of symptoms to prevent acute rheumatic fever, so immediate antibiotic therapy is not crucial if patients can be easily contacted for follow-up should a culture become positive.^[2]

Other

Additional tests include the following:

Mono spot is up to 95% sensitive in children (less than 60% sensitivity in infants)
Peripheral smear may show atypical lymphocytes in infectious mononucleosis ^[3]
Perform gonococcal culture as indicated by history
A complete blood count (CBC), erythrocyte sedimentation rate (ESR), and C-reactive protein have a low predictive value and usually are not indicated ^[30]

Imaging Studies

See the list below:

Imaging studies generally are not indicated for uncomplicated viral or streptococcal pharyngitis.
Lateral neck film should be taken in patients with suspected epiglottitis or airway compromise.
Soft tissue neck CT can be used if concern for abscess or deep-space infection exists; however, peritonsillar abscess is almost always a clinical diagnosis. Imaging is rarely needed for diagnosis.

Procedures

See the list below:

The procedure for a throat swab is to vigorously rub a dry swab over the posterior pharynx and both tonsils, obtaining a sample of exudate. If any exudate is obtained, then transport it dry (not in a liquid medium).

Treatment & Management

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Media Gallery

Streptococcus pyogenes at 100X 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 streptococci.

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Author

John R Acerra, MD Associate Professor, Department of Emergency Medicine, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell; Director, Global Emergency Medicine Program, LIJ Medical Center and North Shore University Hospital, Northwell Health System

John R Acerra, MD is a member of the following medical societies: American Academy of Emergency Medicine, American College of Emergency Physicians, Society for Academic Emergency Medicine

Disclosure: Nothing to disclose.

Specialty Editor Board

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

Disclosure: Received salary from Medscape for employment. for: Medscape.

Chief Editor

Jeter (Jay) Pritchard Taylor, III, MD Assistant Professor, Department of Surgery, University of South Carolina School of Medicine; Attending Physician / Clinical Instructor, Compliance Officer, Department of Emergency Medicine, Prisma Health Richland Hospital

Jeter (Jay) Pritchard Taylor, III, MD is a member of the following medical societies: American Academy of Emergency Medicine, American College of Emergency Physicians, American Medical Association, Columbia Medical Society, Society for Academic Emergency Medicine, South Carolina College of Emergency Physicians, South Carolina Medical Association

Disclosure: Serve(d) as a director, officer, partner, employee, advisor, consultant or trustee for: Employed contractor - Chief Editor for Medscape.

Additional Contributors

Jerry R Balentine, DO, FACEP, FACOEP Vice President, Medical Affairs and Global Health, New York Institute of Technology; Professor of Emergency Medicine, New York Institute of Technology College of Osteopathic Medicine

Jerry R Balentine, DO, FACEP, FACOEP is a member of the following medical societies: American College of Emergency Physicians, New York Academy of Medicine, American College of Osteopathic Emergency Physicians, American Association for Physician Leadership, American Osteopathic Association

Disclosure: Nothing to disclose.

Acknowledgements

Mark W Fourre, MD Associate Clinical Professor, Department of Surgery, University of Vermont School of Medicine; Program Director, Department of Emergency Medicine, Maine Medical Center

Disclosure: Nothing to disclose.