What are the common stool studies for patients presenting with diarrhea?

Common Stool Studies for Diarrhea

Fresh diarrheal stool samples should be tested for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and Shiga toxin-producing E. coli (STEC) in symptomatic patients, with additional testing based on specific risk factors and clinical presentation. 1

Standard Testing Panel

The optimal specimen for laboratory diagnosis is a diarrheal stool sample (one that takes the shape of the container), which provides greater detection rates compared to rectal swabs.

Core Tests:

• Bacterial culture for:
  • Salmonella
  • Shigella
  • Campylobacter
  • Yersinia (especially in school-aged children with right lower quadrant pain)
  • E. coli O157:H7 (using chromogenic agar)
• Shiga toxin testing (to detect non-O157 STEC serotypes)
• C. difficile toxin testing (for patients >2 years with history of antimicrobial use in the previous 8-12 weeks) 1, 2

Additional Testing Based on Clinical Presentation:

For Bloody Diarrhea:

• Priority testing for STEC (both O157:H7 culture and Shiga toxin)
• Blood cultures (if systemic symptoms present) 1, 2

For Travelers:

• Consider parasitic testing for diarrhea lasting ≥14 days
• Test for C. difficile if antimicrobials were used in the preceding 8-12 weeks 1

For Immunocompromised Patients:

• Broader testing including:
  • Cryptosporidium
  • Cyclospora
  • Cystoisospora
  • Microsporidia
  • Mycobacterium avium complex
  • Cytomegalovirus 1

Special Considerations for Testing

When to Test for Specific Pathogens:

• Vibrio species: For patients with large-volume rice-water stools, exposure to brackish/salt water, consumption of raw shellfish, or travel to cholera-endemic regions 1
• Parasites: For travelers with diarrhea lasting >14 days, standard approach includes microscopic examination of 3 stool samples collected on different days 1
• Blood cultures: For infants <3 months, patients with signs of septicemia, suspected enteric fever, systemic manifestations, immunocompromised patients, or travelers from enteric fever-endemic areas 1

Modern Diagnostic Approaches:

• Multiplex molecular panels: These can detect multiple pathogens (bacterial, viral, and parasitic) simultaneously with higher sensitivity than traditional methods 1, 2
• Molecular testing for parasites: Enzyme immunoassay tests (e.g., Giardia antigen) or nucleic acid amplification tests have largely replaced traditional microscopy in many settings 1

Caveats and Pitfalls

1. Interpretation challenges: Molecular tests detect DNA, not necessarily viable organisms, which may lead to false positives 1

2. Unnecessary testing: Routine stool cultures have traditionally been costly and ineffective when used indiscriminately. For hospitalized patients (>3 days), testing for common community pathogens is rarely useful unless there are specific clinical or epidemiological reasons 3

3. C. difficile testing considerations:

   • High rates of asymptomatic colonization in children <2 years make testing results difficult to interpret
   • Only test unformed/diarrheal stools (reject formed stool) 1

4. Specimen quality matters: Fresh stool provides significantly higher detection rates (49%) compared to rectal swabs (9%) for most pathogens 1, 2

5. Consider non-infectious causes: If initial testing is negative or symptoms persist >14 days, consider conditions like inflammatory bowel disease, irritable bowel syndrome, or bile acid malabsorption 1

By following a targeted approach to stool testing based on clinical presentation and risk factors, clinicians can improve diagnostic yield while reducing unnecessary testing and costs.