Streptococcal Disease, Group A - Group A, Strep


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
  • Centor Criteria for Management of Sore Throat

    Centor Criteria for Management of Sore Throat



    Absence of cough


    Swollen and tender AC nodes


    Temperature >100.4ºF (38ºC)


    Tonsillar exudates or swelling




        3-14 years


        15-44 years


         ≥45 years


    Cumulative Score

    Suggested Management


    No testing


    Risk <10%
    Test if high suspicion or recent contact


    Rapid testing or throat culture


    Treatment without testing*

    * Recommendations by Centers for Disease Control (CDC), American College of Physicians (ACP), and American Academy of Family Physicians (AAFP) but not by Infectious Diseases Society of America (IDSA) or American Heart Association (AHA) – approach results in unnecessary prescribing of antibiotics
  • 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 can provide serologic evidence of prior and current infection in suspected poststreptococcal glomerulonephritis or rheumatic fever
    • Not useful in diagnosing acute streptococcal disease (titers rise 7-14 days after infection and peak in 2-3 weeks) 
    • 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

Clinical Background

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

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


  • Antibiotics necessary to eradicate toxin-producing organisms and prevent rheumatic fever and suppurative complications
    • Penicillin still considered drug of choice

Indications for Laboratory Testing

  • 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
Test Name and Number Recommended Use Limitations Follow Up
Streptococcus (Group A) Culture 0060126
Method: Culture/Identification

Gold standard for group A streptococcal infection

Perform if quick testing is negative and suspicion of streptococcus is moderately high

Sensitivity 90-95%

DNase-B Antibody 0050220
Method: Quantitative Nephelometry

Preferred test in most clinical scenarios; best test for rheumatic chorea since it remains elevated longer

Confirm current or recent group A streptococcal infection in patients suspected of having a nonsuppurative complication such as acute glomerulonephritis (AGN) or acute rheumatic fever (ARF)

DNase-B and ASO antibody tests are generally ordered concurrently

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

Confirm a prior infection with group A streptococcus in patients suspected of having a nonsuppurative complication such as AGN or ARF

DNase-B and ASO antibody tests are generally ordered concurrently

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

Additional Tests Available
Click the plus sign to expand the table of additional tests.
Test Name and NumberComments
Streptococcus (Group A) Rapid with Reflex to Culture 0028903
Method: Immunoassay/Culture

If Streptococcus (Group A) Rapid test is negative, culture will be added for confirmation

Streptococcus pyogenes, Group A Antibody (Streptozyme) with Reflex to Titer 0050642
Method: Semi-Quantitative Hemagglutination

Outdated test

Recommend DNase-B antibody and/or Streptolysin O antibody tests