Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1422 page)

   The routine stool culture is used to detect GI infections caused by enteric bacterial pathogens. The target pathogens identified by routine stool culture may vary somewhat among laboratories, but all should be capable of detecting
Salmonella
,
Shigella
, and
Campylobacter
species. Other pathogens may be included, like Shiga toxin–producing
E
.
coli
, depending on local prevalence. Detection of other enteric pathogens may require special testing.

   Method: Stool specimens are typically inoculated onto several agar media, including a nonselective medium (e.g., SBA), a mildly selective differential medium (e.g., MacConkey agar), and a moderately selective differential medium (e.g., Hektoen enteric agar). Some laboratories have used selective broth enrichment (e.g., Selenite broth) prior to plate inoculation, but the cost-effectiveness of these strategies has been questioned. Selective agar media and incubation at increased temperature (42°C) and microaerophilic conditions are used for isolation of enteric
Campylobacter
species.

   
Turnaround time:
Cultures are examined for growth at 24 and 48 hours. Suspicious colonies are isolated and confirmatory testing performed.

   Interpretation

   
Expected results:
Negative.

   
Positive results:
Any growth of
Salmonella
,
Shigella
,
Campylobacter
or other enteric pathogen.

   
Negative results:
The probability infection is decreased but is not excluded. Additional stool cultures or testing for other enteric pathogens should be considered.

   Limitations

   Effective detection of enteric infection due to enterohemorrhagic
E
.
coli
(e.g.,
E
.
coli
O157:H7),
Yersinia enterocolitica
,
Vibrio
species,
Aeromonas
species,
C
.
difficile
, or other bacterial pathogens requires special cultures for sensitive detection.

   Diarrheal illness caused by parasitic or viral pathogens requires special test methods.

   
C
.
difficile
testing should be considered as an alternative to routine enteric pathogen testing for inpatients.

   Stool culture may be negative in invasive enteric infections, like typhoid fever. Blood cultures are recommended for primary gastrointestinal infections that progress to significant fever associated with signs of systemic infection.