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Subject: Throat Culture (Routine)
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This culture is primarily used to detect group A beta-hemolytic Streptococcus (GABHS, S. pyogenes) from throat swabs. This test is used, usually in children, who present with symptoms of streptococcal pharyngitis. Patients typically present with moderate to severe pharyngitis along with systemic symptoms, including fever, malaise, headache, and abdominal pain. Runny nose, cough, diarrhea, and other symptoms are more suggestive of another cause, usually viral.
A GABHS throat culture is recommended to confirm negative S. pyogenes antigen screening tests in children. Confirmatory cultures are not needed for adults with negative antigen test results if the sensitivity of the specific antigen test used is >80%.
The importance of diagnosis of GABHS pharyngitis is for the prevention of nonsuppurative sequelae. Antibiotic treatment given during the acute phase of GABHS infection is effective in prevention of RF, glomerulonephritis, and other complications. GABHS pharyngitis may also be complicated by peritonsillar abscess or other suppurative pararespiratory infections.
A GABHS throat culture is not recommended for test of cure after therapy for documented strep throat; cultures may demonstrate clinically insignificant low-level carriage after successful therapy.
Special collection and transport instructions:
Affected tonsillar and posterior pharyngeal mucosa is rubbed vigorously with a swab, carefully avoiding contamination by the tongue, buccal, or other mucosal surface.
The swab is transported to the laboratory in transport media according to routine recommendations for bacterial specimens.
Throat swabs are inoculated onto SBA; selective agar is inoculated by some laboratories to suppress the growth of normal endogenous flora and to facilitate isolation of GABHS. Cultures are incubated for 24–48 hours.
S. pyogenes isolates remain predictably susceptible to penicillin, the treatment of choice. Antimicrobial susceptibility testing is not performed unless requested because of penicillin allergy.
Turnaround time: Cultures are examined for 24–48 hours. An additional day may be required for isolation and identification of suspected isolates from heavily contaminated specimens.
Expected results: No growth of group A beta-hemolytic Streptococcus.
Positive result: Positive cultures, in the setting of a clinical diagnosis, are diagnostic of GABHS pharyngitis. In the absence of symptoms, positive cultures may indicate carriage and not infection.
Negative results: Throat cultures are sensitive to rule out streptococcal pharyngitis, but may be negative if there is poor specimen collection.
Cultures are typically negative in patients presenting with symptoms consistent with nonsuppurative complications of GABHS infection. Serologic tests, like ASO, may provide support for the diagnosis.
Common pitfall: A throat culture is not optimized for the detection of organisms other than S. pyogenes. (Group C and G beta-streptococci and/or A. hemolyticum are identified in throat cultures in some laboratories.)
Submission of a throat culture is not recommended for detection of carriage or infection by other organisms. To determine the cause of sinusitis or other pararespiratory infections, special procedures for collection and culture (e.g., respiratory tract bacterial culture) are required.
Other causes of pharyngitis include viruses (most common), mycoplasmas, group C and G beta-hemolytic streptococci, and Arcanobacterium hemolyticum. N. gonorrhoeae may be considered in patients at risk. C. diphtheriae is uncommon in the United States, but should be considered in patients at risk. Special testing is usually required to detect pathogens other than S. pyogenes from throat cultures.
GABHS may cause infection at other sites, especially cellulitis. Routine bacterial cultures appropriate for these sites should be requested.