A 10-year-old boy is brought to the office due to abrupt onset fever and sore throat that began a day ago. He has had similar illnesses in the past, which were treated with oral antibiotics. The patient has no other medical conditions and has received all recommended vaccinations. Temperature is 38.3 C (101 F). Physical examination shows erythema of the posterior pharyngeal wall and enlarged tonsils, which are covered by white exudate. There are no enlarged cervical lymph nodes. Throat swab specimen is obtained. Which of the following would be most helpful in determining the need for antibiotic treatment in this patient?
Group A streptococcus (GAS), also called Streptococcus pyogenes, is the leading cause of bacterial pharyngitis in children and adolescents. Manifestations include abrupt-onset fever and sore throat; patients may also have nausea, vomiting, and headache. Examination often reveals exudative tonsillopharyngitis and enlarged tonsils. An alternate diagnosis should be suspected in those with manifestations of viral illness (eg, coryza, conjunctivitis, cough, hoarseness, mouth ulcers).
Patients with suspected GAS pharyngitis require microbiologic testing prior to initiation of antimicrobials. Throat swab samples are collected in-office and are subsequently evaluated by the following:
Rapid antigen detection testing (RADT) – this immunoassay evaluates for GAS antigens and provides rapid, in-office results. However, due to the test's limited sensitivity, patients with negative RADT require throat culture to confirm they do not have the organism.
Throat culture – samples are plated on blood agar and evaluated for beta-hemolysis within 48 hours. GAS is beta-hemolytic due to the expression of cytolysins (eg, streptolysin O), which create a broad zone of complete hemolysis (not incomplete hemolysis) around plated colonies (Choice D).
Patients with positive RADT or throat culture require antibiotic therapy (eg, amoxicillin) to prevent complications of GAS (eg, rheumatic fever), limit transmission to others, and reduce duration/severity of symptoms.
(Choice A) Gram-stain cannot be used to definitively determine if GAS is present because other oropharyngeal organisms (eg, viridans streptococci) also appear as gram-positive cocci in chains.
(Choice B) Antibodies against M protein, a virulence factor that coats the surface of GAS, provide protective immunity against future infections with a particular GAS strain. However, antibody titers take several weeks to elevate after acute infection; therefore, antibody testing is not used to diagnose acute pharyngitis.
(Choice C) Elevated antibody titers against streptolysin O indicate recent GAS infection and can aid the diagnosis of GAS complications (eg, rheumatic fever, glomerulonephritis). However, antibodies against streptolysin take several weeks to form; therefore, they are not used to diagnose acute pharyngitis.
Educational objective:
Group A streptococcus (GAS) should be suspected in those with acute-onset sore throat, exudative tonsillopharyngitis, and no evidence of viral symptoms (eg, coryza, cough, conjunctivitis). In-office throat swab with rapid antigen detection testing (immunoassay for GAS antigens) can provide on-site microbiologic confirmation, allowing for early initiation of treatment.