What stool studies should I order for a patient presenting with diarrhea?

Stool Studies for Diarrhea: A Comprehensive Approach

For patients with diarrhea, stool testing should include bacterial culture for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and Shiga toxin-producing E. coli (STEC), with additional testing based on specific risk factors and clinical presentation. 1

Initial Testing Approach

Acute Diarrhea (≤14 days)

• Standard bacterial pathogens: Test for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and STEC in symptomatic patients 2, 1
• C. difficile testing: Particularly important if antimicrobials were used in the preceding 8-12 weeks 1
  • Only test unformed/diarrheal stools (reject formed stool) 1
  • Not recommended in children <2 years due to high rates of asymptomatic colonization 1
• Bloody diarrhea: Priority testing for STEC (both O157:H7 culture and Shiga toxin) 1
• Blood cultures: Recommended for:
  • Infants <3 months
  • Patients with signs of septicemia
  • Suspected enteric fever
  • Immunocompromised patients
  • Travelers from enteric fever-endemic areas with fever 2, 1

Additional Testing Based on Risk Factors

• Vibrio species: Test if patient has:
  • Large-volume rice-water stools
  • Exposure to brackish/salt water
  • Consumption of raw shellfish
  • Travel to cholera-endemic regions 2, 1
• Yersinia enterocolitica: Test in people with persistent abdominal pain (especially school-aged children with right lower quadrant pain mimicking appendicitis) 2

Chronic Diarrhea (>14 days)

• Giardia testing: Strongly recommended using Giardia antigen test or PCR 2, 1
• Celiac disease screening: Test with IgA tissue transglutaminase and a second test to detect celiac disease in IgA deficiency 2
• Inflammatory markers: Consider fecal calprotectin or fecal lactoferrin to screen for IBD 2
• Bile acid diarrhea testing: Consider in patients with chronic watery diarrhea 2
• Parasitic testing:
  • For travelers with diarrhea lasting >14 days 1
  • Against routine ova and parasite testing in patients with no travel history to or recent immigration from high-risk areas 2

Special Populations

Immunocompromised Patients

• Broader testing: Include viral studies and examination for parasites 2
• For AIDS patients with persistent diarrhea: Additional testing for:
  • Cryptosporidium
  • Cyclospora
  • Cystoisospora
  • Microsporidia
  • Mycobacterium avium complex
  • Cytomegalovirus 2

Outbreak Settings

• Consider broader testing for bacterial, viral, and parasitic agents regardless of fever or bloody stools 2
• Coordinate testing with public health authorities 2, 1

Testing Methods

• Fresh stool samples: Provide significantly higher detection rates (49%) compared to rectal swabs (9%) 1
• Multiplex molecular panels: Can detect multiple pathogens simultaneously with higher sensitivity than traditional methods 1
• Molecular testing for parasites: Enzyme immunoassay tests or nucleic acid amplification tests have largely replaced traditional microscopy 1

Common Pitfalls to Avoid

1. Unnecessary testing: Avoid routine stool cultures and ova/parasite testing in nosocomial diarrhea; focus on C. difficile testing 3
2. Testing formed stools for C. difficile: Only test unformed/diarrheal stools 1
3. Testing for cure: Testing of asymptomatic patients after treatment is not recommended except for epidemiologic purposes 4
4. Delayed testing for STEC: Promptly test for Shiga toxin in bloody diarrhea 1
5. Overlooking non-infectious causes: Consider inflammatory bowel disease, irritable bowel syndrome, or bile acid malabsorption if initial testing is negative or symptoms persist >14 days 1

By following this structured approach to stool studies for diarrhea, you can efficiently identify the causative pathogen while avoiding unnecessary testing, leading to more timely and appropriate treatment decisions.