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]

Overview

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).[1] 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.[1]

Laboratory Findings

Rapid antigen detection test Throat culture Anti–streptococcal antibody titers

Advantages

  • 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.[2]
  • High specificity: RADTs currently available are highly specific (approximately 95%) when compared with blood agar plate cultures.[3]
  • False positive test results are highly unusual, and therefore therapeutic decisions can be made with confidence on the basis of a positive test result.[4]

Disadvantages

  • 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 in adults under usual circumstances.[4]
  • 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 streptococcal infection is suspected and RADT is negative, follow-up with throat culture is warranted due to the possibility of false negative results.
  • RADT has 70% to 90% sensitivity and 90% to 100% specificity.

Advantages

  • High sensitivity: Culture of a single throat swab on a blood agar plate is 90%– 95% sensitive for detection of GAS pharyngitis.

Disadvantages

  • 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.[5]
  • Manner in which the swab is obtained: Throat swab specimens should be obtained from the surface of either tonsils (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

  • Throat culture is the gold standard for the diagnosis of GAS pharyngitis
  • Should be done in adults at high risk for severe infections (immunocompromised patients and those with diabetes mellitus or who use steroids) in whom RADT may be negative.
  • Sensitivity is between 90% and 95%, and specificity is from 95% to 99% when the swab is collected appropriately.
  • Throat culture results will serve as a guide for the completion of treatment.
  • If Neisseria gonorrhoeae is suspected, the diagnosis should be confirmed by culture on Thayer-Martin medium or validated nucleic acid amplification testing.

Advantages

Disadvantages

  • 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 active GAS infection.

Other Laboratory Findings

Other lab tests include

  • Rapid influenza diagnostic tests
    • Immunoassays that can identify the presence of influenza A and B viral nucleoprotein antigens in respiratory specimens
  • Complete blood count with differential
    • An increased percentage of neutrophils may be due to acute bacterial infection
    • An increase in lymphocytes may be related to viral infection
    • Increased total number of lymphocytes, with greater than 10% atypical lymphocytes (large with irregular nuclei) is present in Epston- Bar virus (EBV) infection
    • May be useful when presenting a mononucleosis-type syndrome
  • Monospot test
    • A monospot test (heterophile antibody test) is a rapid test for infectious mononucleosis due to EBV.
  • Epstein-Barr virus serologic profile
    • Serologic profile will include testing for immunoglobulin G (IgG) and M (IgM) antibodies
  • Acute HIV infection tests
    • ELISA test: Uses an enzyme immunoassay to detect specific antibodies

Reference

  1. 1.0 1.1 Shulman ST, Bisno AL, Clegg HW, Gerber MA, Kaplan EL, Lee G et al. (2012) Clinical practice guideline for the diagnosis and management of group A streptococcal pharyngitis: 2012 update by the Infectious Diseases Society of America. Clin Infect Dis 55 (10):1279-82. DOI:10.1093/cid/cis847 PMID: 23091044
  2. 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
  3. 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
  4. 4.0 4.1 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
  5. Schwartz RH, Gerber MA, McCoy P (1985) Effect of atmosphere of incubation on the isolation of group A streptococci from throat cultures. J Lab Clin Med 106 (1):88-92. PMID: 3891893
  6. 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

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