Group A Streptococcal Disease - Strep Throat

Group A Streptococcus, specifically Streptococcus pyogenes, causes various types of infection, most notably acute pharyngitis, also known as strep throat.  Symptoms include sore throat, fever, and painful swallowing.  Although the disease is typically self-limiting, diagnosis is important because treatment shortens the course of the disease and reduces the likelihood of transmitting it to others and developing complications (eg, acute rheumatic fever [ARF] and suppurative complications).   Diagnostic testing includes rapid antigen detection tests (RADTs), nucleic acid amplification tests (NAATs), and throat culture.  

Quick Answers for Clinicians

Which other diseases can be caused by group A Streptococcus?

In addition to strep throat, group A Streptococcus can cause other diseases, such as impetigo, cellulitis, necrotizing fasciitis, and streptococcal toxic shock syndrome.     Diagnosis is often made based on clinical findings and gram stain and culture.    

Which secondary syndromes can follow strep throat or other group A Streptococcus infections?

Scarlet fever is also caused by group A Streptococcus and usually occurs in the presence of strep throat.  It is characterized by an erythematous rash of sandpaper quality, which begins on the trunk and spreads outwardly. Typically, the face, palms, and soles of the feet are not affected.  Testing for scarlet fever is the same as testing for strep throat and includes rapid antigen detection tests (RADTs), nucleic acid amplification tests (NAATs), and throat culture.  

Acute rheumatic fever (ARF) and poststreptococcal glomerulonephritis (PSGN) are secondary syndromes that may follow strep throat or other group A Streptococcus infections.   These are immune-mediated syndromes and are not contagious.   ARF may affect multiple organ systems (eg, cardiovascular, central nervous system, musculoskeletal) and follows 1 to 5 weeks after the onset of strep throat.  Features of PSGN include edema, proteinuria, hypertension, macroscopic hematuria, and lethargy.  PSGN occurs approximately 10 days after strep throat. 

Serology may aid in the diagnosis of these secondary syndromes. 

Indications for Testing

Testing for strep throat should be considered in children 3 years and older and in adolescents who present with a sore throat unless characteristics that suggest a viral etiology (eg, cough, hoarseness, runny nose) are present.  This testing may also be considered in adults with the same clinical presentation. 

Diagnosis

Rapid Antigen Detection Test

The initial test for strep throat is often an RADT.  RADTs have high specificity but varying sensitivity.  A positive RADT is diagnostic for strep throat,  but if RADT results are negative in children, confirmatory testing with a higher sensitivity NAAT or throat culture should be performed.  

Throat Culture

Throat culture is the diagnostic gold standard for strep throat. 

A throat culture should be performed in children and adolescents following a negative RADT.  This is not required for adults due to the low incidence of strep throat and the very low risk of ARF following acute pharyngitis, but throat culture may be beneficial if maximum diagnostic sensitivity is desired. 

Nucleic Acid Amplification Test

A positive RADT may be confirmed by a NAAT.  NAATs have similar sensitivity to throat culture and offer a faster turnaround time.  However, NAATs are more expensive, and unlike throat culture, they cannot detect any other potential bacterial causes of acute pharyngitis.  This testing is not offered at ARUP Laboratories.

Serology

Antibody testing is not recommended for the diagnosis of strep throat because the presence of antibodies indicates a past, not current, infection.  However, detection of antibodies against DNase-B and streptolysin O may aid in the diagnosis of syndromes that may follow strep throat, such as ARF and poststreptococcal glomerulonephritis (PSGN). 

ARUP Laboratory Tests

Culture

Gold standard for confirming group A streptococcal infection

Order for children and adolescents when rapid testing is negative but suspicion of group A Streptococcus remains

Serology

Not useful in the diagnosis of strep throat

May aid in the diagnosis of ARF and PSGN

Medical Experts

Contributor

Fisher

Mark A. Fisher, PhD, D(ABMM)
Associate Professor of Pathology (Clinical), University of Utah
Medical Director, Bacteriology, Special Microbiology, and Antimicrobial Susceptibility Testing, ARUP Laboratories

References