What stool study should be done for a patient presenting with diarrhea?

Stool Studies for Diarrhea: A Diagnostic Approach

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

When to Order Stool Studies

The decision to order stool studies should be based on clinical presentation and risk factors:

• Patients with fever, bloody or mucoid stools, severe abdominal cramping/tenderness, or signs of sepsis should have stool testing for bacterial pathogens 1, 2
• Patients with diarrhea lasting >14 days should be evaluated for parasitic infections 1
• Patients who develop diarrhea after 3 days of hospitalization should be tested for C. difficile rather than routine bacterial pathogens (the "3-day rule") 1
• Patients who have recently taken antibiotics (within 8-12 weeks) should be tested for C. difficile 1

Recommended Stool Studies Based on Clinical Presentation

Acute Diarrhea with Inflammatory Features

• Test for Salmonella, Shigella, Campylobacter, Yersinia, and STEC 1
• For STEC detection, use methods that detect Shiga toxin (or genes that encode them) and distinguish E. coli O157:H7 from other STEC serotypes 1
• Sorbitol-MacConkey agar or appropriate chromogenic agar is recommended to screen for O157:H7 STEC 1

Persistent Diarrhea (14-29 days)

• Test for parasitic infections including Cryptosporidium, Giardia, Cyclospora, and Entamoeba histolytica 1

Hospital-Acquired Diarrhea

• Test primarily for C. difficile toxins in patients hospitalized >3 days 1, 3
• Routine testing for bacterial enteropathogens has very low yield in hospital-acquired diarrhea 1, 4

Special Populations

Immunocompromised Patients

• Perform broader testing including bacterial culture, viral studies, and examination for parasites 1
• For patients with AIDS and persistent diarrhea, test for Cryptosporidium, Cyclospora, Cystoisospora, microsporidia, Mycobacterium avium complex, and cytomegalovirus 1

Travelers with Diarrhea

• Testing is only required for travelers with fever, bloody diarrhea, prolonged course (>5 days), or severe clinical presentation 5
• Test for Campylobacter, Shigella, Salmonella, and parasites (particularly amoebiasis and giardiasis) 5

Diagnostic Methods

Molecular Testing vs. Traditional Methods

• Multiplex PCR panels can detect multiple pathogens simultaneously with high sensitivity and specificity comparable to culture for most pathogens 6
• Traditional stool culture remains important for antimicrobial susceptibility testing and public health surveillance 1
• Specimens that test positive by molecular methods should be cultured if required for public health reporting or antimicrobial susceptibility testing 1

Testing for C. difficile

• C. difficile toxin detection is the primary diagnostic approach 1, 3
• Testing should be performed on unformed stool samples 1
• Consider C. difficile testing in community-acquired diarrhea that is negative for common enteropathogens, regardless of traditional risk factors 1

Common Pitfalls to Avoid

• Testing all diarrheal stools without consideration of clinical presentation leads to unnecessary costs and low yield 1
• Following the "3-day rule" - avoid routine enteric pathogen testing in patients hospitalized >3 days unless there are specific clinical or epidemiological reasons 1
• Failing to test for C. difficile in patients with recent antibiotic exposure 1
• Over-reliance on multiple stool examinations for ova and parasites, which has low yield especially for hospitalized patients 1
• Misinterpreting molecular test results - these detect DNA, not necessarily viable organisms 1

By following these evidence-based recommendations, clinicians can optimize the diagnostic approach to patients with diarrhea while avoiding unnecessary testing and ensuring appropriate management to reduce morbidity and mortality.