When should a stool culture be performed?

When to Perform Stool Culture

Stool cultures should be performed for patients with diarrhea accompanied by fever, bloody or mucoid stools, severe abdominal cramping or tenderness, or signs of sepsis. 1

Specific Indications for Stool Culture

Stool cultures have a relatively low diagnostic yield (1.5-5.6% positivity rate) 1, 2, so they should be ordered selectively based on specific clinical and epidemiological factors:

Clinical Presentations Requiring Stool Culture

• Presence of fever (>37.8°C) - increases odds of positive culture 2.33 times 2
• Bloody or mucoid stools 1
• High-frequency diarrhea (≥5 episodes/day) - increases odds 3.52 times 2
• Severe abdominal cramping or tenderness 1
• Signs of sepsis or systemic illness 1
• Elevated C-reactive protein (>50 mg/L) - increases odds 2.27 times 2
• Persistent diarrhea (>14 days) in travelers 1

Special Patient Populations Requiring Stool Culture

1. Immunocompromised patients:

   • Patients with primary or secondary immune deficiencies
   • People with AIDS (require additional testing for Cryptosporidium, Cyclospora, Cystoisospora, microsporidia, Mycobacterium avium complex, and cytomegalovirus) 1
   • Neutropenic patients 3
   • HIV-infected individuals 3

2. High-risk groups:

   • Infants <3 months of age (blood cultures also indicated) 1
   • Adults ≥65 years with comorbidities 3
   • Patients with hemolytic anemia or other high-risk conditions 1

3. Epidemiological considerations:

   • Recent travel to or contact with travelers from enteric fever-endemic areas 1
   • Exposure to specific risk factors (e.g., raw shellfish consumption for Vibrio) 1
   • Suspected outbreak situations (multiple people with diarrhea sharing a common meal) 1

Special Testing Considerations

Pathogen-Specific Testing

• Yersinia enterocolitica: Test in people with persistent abdominal pain (especially school-aged children with right lower quadrant pain mimicking appendicitis) and those with fever at epidemiologic risk, including infants exposed to raw/undercooked pork 1

• Vibrio species: Test in people with large-volume rice water stools or exposure to salty/brackish waters, consumption of raw/undercooked shellfish, or travel to cholera-endemic regions within 3 days prior to diarrhea onset 1

• Shiga toxin-producing E. coli (STEC): When suspected (based on bloody diarrhea with minimal fever), use diagnostic approaches that detect Shiga toxin and distinguish E. coli O157:H7 from other STEC 1, 4

The "3-Day Rule" for Hospitalized Patients

Stool cultures have very low yield when performed on patients who develop diarrhea after 3 days of hospitalization, except in these circumstances 1, 3:

• Age >65 years with comorbidities
• Immunocompromised status (neutropenia, HIV)
• Suspected nosocomial outbreak
• Patient not previously cultured for indicated pathogens

For hospital-acquired diarrhea (>3 days after admission), C. difficile testing is more appropriate than standard bacterial cultures 1.

When Stool Culture is NOT Recommended

• Uncomplicated traveler's diarrhea unless treatment is indicated 1
• Patients with vomiting as predominant symptom (negative predictor for bacterial pathogens) 2
• Routine testing of hospitalized patients with diarrhea developing >3 days after admission (unless meeting special criteria above) 1, 3
• Patients with uncomplicated, mild, non-bloody diarrhea without fever 1, 4

Optimal Specimen Collection

The optimal specimen for laboratory diagnosis is a diarrheal stool sample that takes the shape of the container. If timely collection isn't possible, a rectal swab may be used for bacterial detection, though with lower sensitivity. Fresh stool is preferred for viral, protozoal, and C. difficile testing 1.

Cost-Effectiveness Considerations

Implementing selective criteria for stool cultures can reduce unnecessary testing by up to 52% without missing clinically significant cases 3. Using the clinical predictors of fever, high-frequency diarrhea (≥5/day), and elevated CRP (>50 mg/L) can help identify patients most likely to have positive cultures 2.