What stool studies labs should be ordered for patients presenting with gastrointestinal symptoms?

Stool Studies for Patients with Gastrointestinal Symptoms

Stool testing should be performed for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and Shiga toxin-producing E. coli (STEC) in patients with diarrhea accompanied by fever, bloody or mucoid stools, severe abdominal cramping or tenderness, or signs of sepsis. 1

Patient Selection for Stool Testing

Indications for Comprehensive Stool Testing

• Acute diarrhea with alarm features:

  • Fever
  • Bloody or mucoid stools
  • Severe abdominal cramping or tenderness
  • Signs of sepsis 1, 2

• High-risk populations:

  • Infants <3 months of age
  • Immunocompromised patients
  • Patients with hemolytic anemia or other high-risk conditions
  • Recent travelers from endemic areas with febrile illness 1

• Epidemiological risk factors:

  • Recent travel to endemic regions
  • Exposure to contaminated food or water
  • Outbreak settings
  • Healthcare-associated diarrhea 1, 2

When Testing Can Be Limited or Deferred

• Uncomplicated traveler's diarrhea (unless symptoms persist >14 days) 1
• Mild, self-limited diarrhea without alarm features
• Suspected functional disorders (IBS) without alarm features 1

Specific Testing Based on Clinical Presentation

1. Acute Diarrhea (<14 days)

Standard testing panel:

• Bacterial culture for Salmonella, Shigella, Campylobacter
• C. difficile testing (if healthcare exposure or recent antibiotics)
• STEC testing (culture for O157:H7 and Shiga toxin assays) 1, 2

Additional targeted testing based on presentation:

• For bloody diarrhea: Prioritize STEC testing with both culture for O157:H7 and Shiga toxin assays 1
• For rice-water stools or seafood exposure: Add Vibrio species testing 1
• For persistent abdominal pain (especially in school-aged children): Add Yersinia enterocolitica testing 1
• For suspected outbreak: Consider broader testing for bacterial, viral, and parasitic agents 1, 2

2. Persistent Diarrhea (14-29 days)

• Standard bacterial pathogens plus:
• Parasitic testing (including Giardia, Cryptosporidium)
• Consider testing for:
  • Entamoeba histolytica
  • Cyclospora cayetanensis 1, 3

3. Chronic Diarrhea (≥30 days)

• Categorize as watery, fatty, or inflammatory using stool studies:
  • Fecal calprotectin or lactoferrin (inflammatory markers)
  • Fecal fat (for malabsorption)
  • Stool osmolality, pH, electrolytes (for osmotic vs. secretory diarrhea) 4, 3
• Consider:
  • Bile acid malabsorption testing
  • Pancreatic elastase (for exocrine pancreatic insufficiency)
  • Celiac disease serologies (anti-tissue transglutaminase IgA, total IgA) 4

4. Immunocompromised Patients

• Broader testing for:
  • Standard bacterial pathogens
  • Viral studies (including CMV)
  • Extended parasitic testing including:
    • Cryptosporidium
    • Cyclospora
    • Cystoisospora
    • Microsporidia
    • Mycobacterium avium complex 1, 2

Specimen Collection Best Practices

• Preferred specimen: Fresh diarrheal stool that takes the shape of the container 2
• Timing: Collect during acute phase of illness before antimicrobial therapy when possible
• Alternative: Rectal swab if timely diarrheal stool cannot be collected (note: reduced sensitivity, especially for parasites at 57.1%) 2
• Multiple specimens: Generally not necessary; a single comprehensive examination is sufficient in most cases 2

Testing Methods

Bacterial Pathogens

• Culture-based methods or nucleic acid amplification tests (NAAT)
• For STEC: Sorbitol-MacConkey agar or chromogenic agar for O157:H7; Shiga toxin testing for non-O157 STEC 1, 2

C. difficile

• NAAT for toxin genes
• GDH antigen with toxin detection
• Test only on unformed stools in patients >2 years with relevant history 2

Parasitic Testing

• Ova and parasite examination with permanent stained smears
• Specific immunoassays for Giardia, Cryptosporidium
• NAAT for parasitic detection 2

Common Pitfalls and Caveats

• Overreliance on multiplex panels: While gastrointestinal pathogen panels (GPPs) have higher pathogen detection rates, they may lead to overdiagnosis of colonization rather than true infection, potentially resulting in inappropriate management and higher healthcare utilization 5

• Testing after antibiotics: May reduce sensitivity for bacterial pathogens

• Delayed testing in chronic symptoms: For chronic diarrhea, categorizing the type (inflammatory, fatty, osmotic, or secretory) is more useful than focusing only on infectious causes 4, 3

• Failure to consider non-infectious causes: Most cases of chronic diarrhea are non-infectious, with IBS/functional diarrhea being the most common cause 4

• Inappropriate C. difficile testing: Test only unformed stools in patients with appropriate risk factors 2

By following this structured approach to stool studies based on clinical presentation and risk factors, clinicians can efficiently diagnose and manage patients with gastrointestinal symptoms while avoiding unnecessary testing.