What is the best course of action for ongoing diarrhea and do I need to order a specific stool sample test?

Stool Testing for Ongoing Diarrhea

Yes, you need to order specific tests based on the duration and characteristics of the diarrhea—a single diarrheal stool specimen should be sent for bacterial culture (Salmonella, Shigella, Campylobacter, Yersinia, STEC), C. difficile testing, and Giardia antigen or PCR. 1, 2

Optimal Specimen Collection

• Collect a diarrheal stool sample that takes the shape of the container—this is the optimal specimen for laboratory diagnosis of infectious diarrhea 1
• If a timely diarrheal stool sample cannot be collected, a rectal swab may be used for bacterial detection, though fresh stool is preferred for viral, protozoal agents, and C. difficile toxin 1
• A single specimen is sufficient for most testing 1, 2

Core Testing Panel for Ongoing Diarrhea

Bacterial Pathogens

• Order stool culture for Salmonella, Shigella, Campylobacter, and Yersinia 1, 2
• Test specifically for Shiga toxin-producing E. coli (STEC) using either sorbitol-MacConkey agar for O157:H7 or Shiga toxin detection for other serotypes 1
• Consider Yersinia testing specifically if the patient has persistent abdominal pain (especially right lower quadrant pain mimicking appendicitis) or exposure to raw/undercooked pork 1

Parasitic Testing

• Always test for Giardia using antigen test or PCR—this is a strong recommendation as Giardia is a common cause of persistent diarrhea with excellent diagnostic test performance (>95% sensitivity and specificity) 1, 2
• Do NOT routinely order ova and parasite examination unless the patient has travel history to or recent immigration from high-risk areas 1

C. difficile

• Test for C. difficile if the patient has recent antimicrobial use (within 8-12 weeks), healthcare-associated diarrhea, or persistent diarrhea without identified etiology 1, 2

Additional Laboratory Work

• Order CBC and basic metabolic panel to assess for inflammation, dehydration, and electrolyte abnormalities 2

Special Circumstances Requiring Modified Testing

Bloody or Mucoid Stools

• Prioritize testing for STEC, Shigella, Salmonella, Campylobacter, Yersinia, and Entamoeba histolytica 1, 2
• These pathogens are the most likely causes of visible blood in stool 1

Immunocompromised Patients

• Order a broad differential including culture, viral studies, and parasitic examination 1
• For AIDS patients with persistent diarrhea, add testing for Cryptosporidium, Cyclospora, Cystoisospora, microsporidia, Mycobacterium avium complex, and cytomegalovirus 1

Travel History

• If diarrhea persists ≥14 days after travel, evaluate for intestinal parasitic infections including ova and parasite examination 1, 2
• Test for Vibrio species if there is exposure to salty/brackish waters, consumption of raw/undercooked shellfish, or travel to cholera-endemic regions within 3 days of symptom onset 1

Persistent Diarrhea (≥14 Days)

• If initial testing is negative and symptoms persist beyond 14 days, consider non-infectious causes including inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS) 1, 2
• Reassess fluid and electrolyte balance, nutritional status, and consider lactose intolerance 1

What NOT to Order

• Do NOT order fecal leukocytes or stool lactoferrin—these should not be used to establish the cause of acute infectious diarrhea 1
• Do NOT order routine ova and parasite examination without travel history or immigration from high-risk areas 1
• Do NOT order serologic tests for enteric fever—these are not recommended 1

Common Pitfalls to Avoid

• Avoid ordering comprehensive "ova and parasite" panels in patients without travel history—this is low yield and not cost-effective 1
• When using multiplex molecular diagnostic panels, remember these detect DNA and not necessarily viable organisms, so clinical context is essential for interpretation 1
• Do not repeat stool cultures for case management after symptom resolution unless required by public health authorities for specific pathogens (Salmonella Typhi, STEC, Shigella) in food handlers or childcare workers 1