What is the appropriate stool test for a patient with gastrointestinal symptoms?

Appropriate Stool Testing for Patients with Gastrointestinal Symptoms

For patients with gastrointestinal symptoms, stool testing should be targeted based on clinical presentation, with a multistep algorithm for C. difficile testing and selective testing for other pathogens based on specific risk factors and symptom patterns.

Clinical Decision Algorithm for Stool Testing

Initial Assessment

• Test selection should be based on:
  • Duration of symptoms
  • Presence of fever, bloody or mucoid stools
  • Severe abdominal cramping or tenderness
  • Signs of sepsis
  • Recent antibiotic use
  • Healthcare exposure
  • Travel history
  • Immune status

Specific Testing Recommendations

For Patients with Acute Diarrhea (≥3 loose stools in 24 hours)

1. C. difficile Testing:

   • Test for C. difficile in patients with:
     • Recent antibiotic exposure (within 8-12 weeks) 1
     • Healthcare-associated diarrhea 1
     • Persistent diarrhea without other etiology 1
   • Use a multistep algorithm rather than NAAT alone:
     • GDH plus toxin; or
     • GDH plus toxin, arbitrated by NAAT; or
     • NAAT plus toxin 1
   • Do not test formed stools or repeat testing within 7 days 1
   • Do not test in children <12 months due to high asymptomatic carriage 1

2. Bacterial Pathogens Testing:

   • Test for Salmonella, Shigella, Campylobacter, Yersinia, and STEC in patients with:
     • Fever
     • Bloody or mucoid stools
     • Severe abdominal cramping
     • Signs of sepsis 1
   • Test for Vibrio species in patients with:
     • Large volume rice water stools
     • Exposure to brackish waters
     • Consumption of raw/undercooked shellfish
     • Travel to cholera-endemic regions 1

3. Parasitic Testing:

   • Restrict ova and parasite (O&P) testing to patients with:
     • Diarrhea lasting >14 days 1
     • Travel to endemic areas 1, 2
     • Immunocompromised status (especially HIV) 1, 2
     • Prior parasitic disease 2
     • Institutionalization 2
   • For travelers with persistent diarrhea (>14 days), evaluate for intestinal parasitic infections 1

4. Blood Cultures:

   • Obtain blood cultures in:
     • Infants <3 months of age
     • Patients with signs of septicemia
     • Suspected enteric fever
     • Immunocompromised patients
     • Patients with hemolytic anemia
     • Travelers from enteric fever-endemic areas with febrile illness 1

Special Populations

1. Immunocompromised Patients:

   • Perform broader testing including:
     • Bacterial culture
     • Viral studies
     • Parasitological examination 1
   • In AIDS patients with persistent diarrhea, test additionally for:
     • Cryptosporidium
     • Cyclospora
     • Cystoisospora
     • Microsporidia
     • Mycobacterium avium complex
     • Cytomegalovirus 1

2. Hospitalized Patients:

   • Avoid routine O&P testing after 3 days of hospitalization (very low yield) 3, 4
   • For nosocomial diarrhea, focus on C. difficile testing rather than routine bacterial cultures or O&P 3

3. Outbreak Settings:

   • Consider broader testing regardless of clinical presentation
   • Coordinate with public health authorities 1

Specimen Collection Best Practices

• The optimal specimen is a diarrheal stool sample (takes shape of container) 1
• For bacterial infections, if timely stool collection is impossible, rectal swabs may be used 1
• For patients with ileus who cannot produce stool, perirectal swabs can be tested by PCR for C. difficile 1
• Transport specimens promptly to the laboratory or refrigerate if delay >1 hour 1

Common Pitfalls to Avoid

1. Ordering stool O&P examinations routinely for all diarrhea cases (yield is only ~2%) 2
2. Testing for C. difficile in patients receiving laxatives 1
3. Repeat testing for C. difficile within 7 days during the same episode 1
4. Testing formed stools for infectious pathogens 1
5. Failing to test for C. difficile in patients with recent antibiotic exposure 1
6. Ordering comprehensive stool cultures for nosocomial diarrhea without C. difficile testing 3

By following this evidence-based approach to stool testing, clinicians can maximize diagnostic yield while minimizing unnecessary testing, leading to better patient outcomes and resource utilization.