Pharyngitis laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Venkata Sivakrishna Kumar Pulivarthi M.B.B.S [2]


Testing for pharyngitis usually is not recommended for children or adults with acute pharyngitis with clinical and epidemiological features that strongly suggest a viral etiology (eg, cough, rhinorrhea, hoarseness, and oral ulcers). Diagnostic studies for GAS are not indicated for children < 3 years old because acute rheumatic fever is rare in these and the incidence of streptococcal pharyngitis and the classic presentation of streptococcal pharyngitis are uncommon in this age group. Selected children < 3 years old who have other risk factors, such as an older sibling with GAS infection, may be considered for testing.

Laboratory Findings

Rapid antigen detection test Throat culture Anti–streptococcal antibody titers


  • Rapidity of the test: Rapid identification and treatment of patients with GAS pharyngitis can reduce the risk of spread, allowing the patient to return to school or work sooner, and can reduce the acute associated morbidity.[1]
  • High specificity: RADTs currently available are highly specific (approximately 95%) when compared with blood agar plate cultures.[2]
  • False positive test results are highly unusual, and therefore therapeutic decisions can be made with confidence on the basis of a positive test result.[3]


  • Sensitivity is low: Because the sensitivities of the various RADTs are <90% and because the proportion of acute pharyngitis due to GAS in children and adolescents is sufficiently high (20%-30%), a negative RADT should be accompanied by a follow-up or back-up throat culture in children and adolescents, while this is not necessary for adults under usual circumstances.[3]
  • Can not differentiate acutely infected persons from asymptomatic streptococcal carriers with intercurrent viral pharyngitis.

Description about the test

  • Adults with 2 or more Centor criteria should have RADT
  • A positive RADT establishes the diagnosis for GAS pharyngitis in conjunction with supportive clinical and epidemiological evidence.
  • If RADT is positive but is not associated with clinical evidence of infection, it identifies a Streptococcus carrier who is chronically colonized.
  • If the streptococcal infection is suspected and RADT is negative, follow-up with a throat culture is warranted due to the possibility of false-negative results.
  • RADT has 70% to 90% sensitivity and 90% to 100% specificity.



  • A major disadvantage of throat cultures is the delay (overnight or longer) in obtaining results.
  • Can not differentiate acutely infected persons from asymptomatic streptococcal carriers with intercurrent viral pharyngitis.

Variables that affect culture results

  • Culture methods: Use of anaerobic incubation and selective culture media may increase the proportion of positive culture results.[4]
  • Manner in which the swab is obtained: Throat swab specimens should be obtained from the surface of either tonsil (or tonsillar fossae) and the posterior pharyngeal wall. Other areas of the oral pharynx and mouth are not acceptable sites. An uncooperative child without immobilizing the neck may obtain a specimen that is neither adequate nor representative.
  • Duration of Incubation

Description about the test



  • Testing of antibody is not useful in the diagnosis of acute pharyngitis because antibody titers of the 2 most commonly used tests, antistreptolysin O (ASO) and anti- DNase B, may not reach maximum levels until 3–8 weeks after acute GAS pharyngeal infection and may remain elevated for months even without an active GAS infection.

Other Laboratory Findings

Other lab tests include


  1. Randolph MF, Gerber MA, DeMeo KK, Wright L (1985) Effect of antibiotic therapy on the clinical course of streptococcal pharyngitis. J Pediatr 106 (6):870-5. PMID: 3923180
  2. Gerber MA (1989) Comparison of throat cultures and rapid strep tests for diagnosis of streptococcal pharyngitis. Pediatr Infect Dis J 8 (11):820-4. PMID: 2687791
  3. 3.0 3.1 Gerber MA, Shulman ST (2004) Rapid diagnosis of pharyngitis caused by group A streptococci. Clin Microbiol Rev 17 (3):571-80, table of contents. DOI:10.1128/CMR.17.3.571-580.2004 PMID: 15258094
  4. Schwartz RH, Gerber MA, McCoy P (1985) Effect of the atmosphere of incubation on the isolation of group A streptococci from throat cultures. J Lab Clin Med 106 (1):88-92. PMID: 3891893
  5. Shet A, Kaplan EL (2002) Clinical use and interpretation of group A streptococcal antibody tests: a practical approach for the pediatrician or primary care physician. Pediatr Infect Dis J 21 (5):420-6; quiz 427-30. PMID: 12150180