Streptococcal Disease, Group A - Group A, Strep

  • Diagnosis
  • Background
  • Lab Tests
  • References
  • Related Topics

Indications for Testing

  • Sore throat, fever, and lymphadenopathy

Laboratory Testing

  • Clinical diagnosis unreliable
  • Consider Centor score when testing for group A strep in patient with sore throat
  • Rapid antigen detection with reflex to culture for initial negative antigen testing in children (rapid antigen tests are 70-90% sensitive; specificity >95%)
  • Swabbing post pharynx increases sensitivity of both rapid antigen test and culture
  • Positive rapid test confirms strep infection in presence of clinical syndrome
  • Throat culture – sensitivity 90-95%
  • Occurrence of strep infection in adults is low (10%); follow-up culture of negative rapid antigen not usually indicated
  • Antibody tests (ASO and anti-DNase-B) can provide serologic evidence of prior and current infection in suspected poststreptococcal glomerulonephritis or rheumatic fever (Parks, 2015)
    • May be indicated for
      • Patients with suspected acute rheumatic fever and poststreptococcal glomerulonephritis in high risk settings
        • Either titer (ASO, anti-DNase-B) may be used for glomerulonephritis antibody
        • Both antibody tests should be used to exclude rheumatic heart disease
      • Patients with suspected rheumatic fever and poststreptococcal pharyngitis in low risk settings
        • ASO usually sufficient to rule out disease
    • Not useful in diagnosing acute streptococcal disease (titers rise 7-14 days after infection and peak in 2-3 weeks)
      •  Convalescent titers with rise best evidence of antecedent streptococcal infection
  • Elevated serum antistreptolysin O titers are found in only about 85% of individuals with acute rheumatic fever
    • When both ASO and anti-DNase-B antibodies are detected, 90-95% of acute rheumatic fever patients have elevated titers
    • Skin infections with group A streptococci are often associated with a poor antistreptolysin O response, but anti-DNase-B titers are increased after skin infection

Differential Diagnosis

Group A streptococci (Streptococcus pyogenes) cause a broad spectrum of disease and are the primary bacterial cause of exudative tonsillitis and pharyngitis.


  • Prevalence – streptococcal pharyngitis represents 1-2% of primary care visits and is one of the top 20 reported diseases
    • Incidence of rheumatic fever poststreptococcal pharyngitis – 1-2/100,000
  • Age – usually <20 years
    • Rare in children <3 years old
  • Transmission – direct contact with nasal discharge
    • Seasonally most common in winter and/or early spring


  • Streptococcus pyogenes bacteria are gram-positive cocci
  • Organism may evoke cross-reacting immune response with human tissue leading to the postinfection sequelae of acute rheumatic fever (including rheumatic heart disease) and acute glomerulonephritis
    • Acute rheumatic fever and rheumatic heart disease are associated with streptococcal pharyngitis
    • Acute glomerulonephritis is associated with both streptococcal pharyngitis and skin disease
  • Causes a range of human disease
    • Pharyngitis
    • Impetigo
    • Scarlet fever
    • Invasive infections

Risk Factors

  • Young age
  • Crowded conditions

Clinical Presentation

  • Primary symptoms – sudden onset sore throat, fever (>100.4°F), headache
  • Constitutional symptoms – nausea, vomiting, abdominal pain
  • Clinical findings – pharyngeal erythema, exudative tonsillitis, anterior cervical adenopathy, palatine petechiae
    • In the absence of above symptoms, the presence of cough, hoarseness, pharyngeal ulcers, diarrhea, and/or conjunctivitis are more suggestive of viral pharyngitis
    • Presence of finely papular erythematous rash – scarlet fever
  • Complications
    • Peritonsillar abscess
    • Toxic shock syndrome
    • Rheumatic fever
      • Rheumatic heart disease
    • Poststreptococcal glomerulonephritis
    • Poststreptococcal reactive arthritis
Tests generally appear in the order most useful for common clinical situations. Click on number for test-specific information in the ARUP Laboratory Test Directory.

Streptococcus (Group A) Culture 0060126
Method: Culture/Identification

DNase-B Antibody 0050220
Method: Quantitative Nephelometry


Antibiotic therapy decreases antibody levels


May want to order a throat culture at the same time

Recommend testing acute and convalescent samples taken approximately 2 weeks apart

Antibodies may persist for 2-3 months after infection

Streptolysin O Antibody (ASO) 0050095
Method: Quantitative Nephelometry


Not helpful in the diagnosis of the agent of skin infections

Antibiotic therapy decreases elevation of test


May want to order a throat culture at the same time

Recommend testing acute and convalescent samples taken approximately 2 weeks apart

Antibodies may persist for 6 weeks after infection


Shulman ST, Bisno AL, Clegg HW, Gerber MA, Kaplan EL, Lee G, Martin JM, Van Beneden C, Infectious Diseases Society of America. 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. 2012; 55(10): e86-102. PubMed

General References

Alcaide ML, Bisno AL. Pharyngitis and epiglottitis. Infect Dis Clin North Am. 2007; 21(2): 449-69, vii. PubMed

Choby BA. Diagnosis and treatment of streptococcal pharyngitis. Am Fam Physician. 2009; 79(5): 383-90. PubMed

Clerc O, Greub G. Routine use of point-of-care tests: usefulness and application in clinical microbiology. Clin Microbiol Infect. 2010; 16(8): 1054-61. PubMed

Kociolek LK, Shulman ST. In the clinic. Pharyngitis. Ann Intern Med. 2012; 157(5): ITC3-1 - ITC3-16. PubMed

Parks T, Smeesters PR, Curtis N, Steer AC. ASO titer or not? When to use streptococcal serology: a guide for clinicians. Eur J Clin Microbiol Infect Dis. 2015; 34(5): 845-9. PubMed

Wessels MR. Clinical practice. Streptococcal pharyngitis. N Engl J Med. 2011; 364(7): 648-55. PubMed

References from the ARUP Institute for Clinical and Experimental Pathology®

Martins TB, Augustine NH, Hill HR. Development of a multiplexed fluorescent immunoassay for the quantitation of antibody responses to group A streptococci. J Immunol Methods. 2006; 316(1-2): 97-106. PubMed

Martins TB, Hoffman JL, Augustine NH, Phansalkar AR, Fischetti VA, Zabriskie JB, Cleary PP, Musser JM, Veasy G, Hill HR. Comprehensive analysis of antibody responses to streptococcal and tissue antigens in patients with acute rheumatic fever. Int Immunol. 2008; 20(3): 445-52. PubMed

Martins TB, Veasy G, Hill HR. Antibody responses to group A streptococcal infections in acute rheumatic fever. Pediatr Infect Dis J. 2006; 25(9): 832-7. PubMed

Stockmann C, Ampofo K, Hersh AL, Blaschke AJ, Kendall BA, Korgenski K, Daly J, Hill HR, Byington CL, Pavia AT. Evolving epidemiologic characteristics of invasive group a streptococcal disease in Utah, 2002-2010. Clin Infect Dis. 2012; 55(4): 479-87. PubMed

Medical Reviewers

Last Update: October 2017