Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1430 page)

   
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.

   Interpretation

   
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.

   Limitations

   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 Considerations

   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.