Recommended Stool Studies for Diagnosing the Cause of Diarrhea
Stool testing should be performed selectively based on specific clinical presentations, with testing for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and STEC in symptomatic patients with appropriate risk factors or clinical features. 1
When to Order Stool Studies
Stool studies should not be ordered routinely for all cases of diarrhea. Instead, testing should be targeted based on:
Clinical Presentations Warranting Stool Testing:
- Bloody or mucoid stools - Test for STEC, Shigella, Salmonella, Campylobacter, E. histolytica 1
- Severe abdominal pain with minimal fever - Consider STEC, Salmonella, Shigella, Campylobacter 1
- Persistent diarrhea (>14 days) - Test for parasites (Cryptosporidium, Giardia, Cyclospora) 1
- Recent antibiotic use - Test for C. difficile (in patients >2 years old) 1
- Immunocompromised status - Broader testing for bacterial, viral, and parasitic pathogens 1
- Traveler's diarrhea lasting >14 days - Test for intestinal parasites 1
- Outbreak situations - Broader testing based on epidemiologic factors 1
Recommended Stool Studies by Clinical Scenario
1. Acute Diarrhea with Blood or Mucus:
- Bacterial culture for Salmonella, Shigella, Campylobacter
- STEC testing (culture on sorbitol-MacConkey agar for O157:H7 and Shiga toxin detection for non-O157 strains)
- Consider E. histolytica testing in appropriate epidemiologic settings
2. Persistent Diarrhea (>14 days):
- Ova and parasite examination for Giardia, Cryptosporidium, Cyclospora
- Consider inflammatory bowel disease and irritable bowel syndrome in differential diagnosis 1
3. Healthcare-Associated or Antibiotic-Associated Diarrhea:
- C. difficile toxin testing (only in patients >2 years old) 1
4. Immunocompromised Patients:
- Comprehensive testing including:
- Bacterial culture
- Viral studies
- Parasite examination
- Additional testing for Cryptosporidium, Cyclospora, Cystoisospora, microsporidia, Mycobacterium avium complex, and cytomegalovirus in AIDS patients 1
5. Traveler's Diarrhea:
- Testing generally not recommended for uncomplicated cases
- For persistent cases (>14 days), test for intestinal parasites
- Consider C. difficile testing if antibiotics were used in preceding 8-12 weeks 1
Specimen Collection Guidelines
- Optimal specimen: Fresh diarrheal stool sample (takes shape of container)
- If timely collection is impossible, rectal swab may be used for bacterial detection (less sensitive)
- Fresh stool is preferred for viral, protozoal agents, and C. difficile toxin testing 1
Tests to Avoid
- Fecal leukocyte examination and stool lactoferrin detection should not be used to establish the cause of acute infectious diarrhea (strong recommendation) 1
- These tests have limited diagnostic value compared to specific pathogen testing
Follow-up Testing
- Follow-up testing after resolution of diarrhea is generally not recommended for clinical management
- Exception: Public health requirements for certain pathogens (Salmonella Typhi/Paratyphi, STEC, Shigella) 1
- For patients who don't respond to initial therapy, consider:
- Reassessment for non-infectious causes (lactose intolerance, IBD, IBS)
- Re-evaluation of fluid/electrolyte balance and nutritional status 1
Special Considerations
Blood cultures should be obtained 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
Imaging studies (ultrasound, CT, MRI) may be considered in older patients with invasive Salmonella or Yersinia infections with persistent fever or bacteremia 1
By following these evidence-based guidelines for stool studies, clinicians can effectively diagnose the cause of diarrhea while avoiding unnecessary testing, leading to improved patient outcomes through appropriate and timely treatment.