Diagnostic Testing Shows Group A Streptococcus

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A 28-year-old male presented to the Emergency Department (ED) with a 1-day history of sore throat and fever. Upon physical examination, there was tonsillar swelling noted without the presence of pus. A throat swab was collected and a rapid antigen detection test (RADT) for Group A Streptococcus (GAS) was performed in the ED. Results were negative. A throat swab for culture was also collected. The patient was sent home with a diagnosis of presumed viral pharyngitis. The next day the patient returned to the ED with worsening sore throat and difficulty swallowing. Pus was seen on the tonsils and the uvula is deviated towards one side of his throat. 

Lab Testing

The throat was cultured for Group A Streptococcus. The sample was plated on a Blood Agar Plate (BAP) and incubated for 24 hours prior to reading the culture. A Gram stain was performed and showed gram-positive cocci in chains. The isolate was catalase negative and susceptible to bacitracin. Based on the laboratory culture, the organism was identified as Group A Streptococcus or Streptococcus pyogenes. It is important to note that RADT is only 55-85% sensitive, while the throat culture performed in the microbiology laboratory is ~95% sensitive. If a RADT is performed and the result is negative, a GAS throat culture should be performed to rule out GAS infection. A positive RADT does not need to be confirmed since the specificity of the test is very high. 

Cause of Symptoms

Many organisms can cause acute pharyngitis. Approximately 60% of cases are caused by viral agents such as rhinovirus, adenovirus, coronavirus, Epstein-Barr virus and others. Approximately 15% of cases are caused by bacterial agents such as Group A Streptococcus, Group C and G Streptococcus, Archanobacterium haemolyticum, Fusobacterium nucleatum, Corynebacterium diphtheriae, Neisseria gonorrhoeae and others. Remaining cases are of unknown or non-infectious etiology. 


Treatment for GAS is typically penicillin. 

Contact Information

Nicole Jackson, njackson@asmusa.org