Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1348 page)

   This test is a specialized stool culture for the detection of GI infection caused by
Escherichia coli
strains associated with enterohemorrhagic infection. These strains produce a Shiga toxin and are most commonly, but not exclusively, associated with
E
.
coli
O157:H7 strains. Enterohemorrhagic
E
.
coli
O157:H7 gastroenteritis commonly presents with abdominal pain with vomiting and diarrhea. Stool may become bloody with signs of colitis. Low-grade fever may be present. In most patients, the symptoms resolve within a week. Rare patients, usually the elderly or very young patients, develop HUS with onset commonly occurring 7 days or more after onset of diarrheal symptoms.

   Use

   This culture is used to diagnose GI infection caused by Shiga toxin–producing
E
.
coli
. (Stool may be tested directly for the presence of Shiga toxin as an alternative to culture isolation). Special medium (sorbitol–MacConkey agar) is used to screen stool. Suspicious isolates are confirmed by serotyping and/or Shiga toxin production.
E
.
coli
O157:H7 strains are almost all sorbitol negative.

   
Turnaround time:
24–48 hours. Additional time is needed for positive cultures to confirm final identification.

   Special Collection and Transport Instructions

   Specimens are collected and transported according to recommendation for routine stool culture.

   Interpretation

   
Expected results:
Negative.

   
Negative results:
Infection is unlikely, but a single-negative culture does not rule out infection by enterohemorrhagic
E
.
coli
.

   
Positive results:
A positive test indicates infection by
E
.
coli
O157:H7 in patients with a compatible clinical presentation.

   Limitations

   Cultures are usually positive only in acute, early infection. The use of stool culture for evaluation of patients with HUS is limited. The use of sorbitol–MacConkey agar is not sensitive for the detection of non–O157 Shiga toxin–producing strains of
E
.
coli
; alternative testing methods should be used in areas where toxigenic non-O157 strains are prevalent or during outbreaks caused by non-O157 strains. Antibiotic therapy of
E
.
coli
O157:H7 infection is not routinely recommended; treatment may induce Shiga toxin production and increase disease severity.