What lab tests should be ordered for a patient with diarrhea lasting 2 weeks?

Laboratory Tests for Diarrhea Lasting 2 Weeks

For a patient with diarrhea lasting 2 weeks, stool testing should be performed for Salmonella, Shigella, Campylobacter, Yersinia, Clostridium difficile, and Shiga toxin-producing E. coli (STEC), as well as examination for parasites. 1

Initial Laboratory Evaluation

• Complete blood count (CBC), C-reactive protein (CRP), basic metabolic panel (BMP) to assess for inflammation, dehydration, and electrolyte abnormalities 1, 2
• Anti-tissue transglutaminase IgA and total IgA to screen for celiac disease 2
• Thyroid function tests to rule out hyperthyroidism as a cause of chronic diarrhea 1
• Stool culture for bacterial pathogens (Salmonella, Shigella, Campylobacter, Yersinia) 1
• Testing for C. difficile toxin, especially if there is history of antibiotic use within the preceding 8-12 weeks 1
• Stool examination for ova and parasites, particularly for Giardia, Cryptosporidium, Cyclospora, and Entamoeba histolytica 1

Stool Characteristics and Specific Testing

For Watery Diarrhea

• Stool osmotic gap to differentiate secretory from osmotic diarrhea 2, 3
• Stool electrolytes (Na+, K+) 1
• Consider 75Se-HCAT scan or serum 7α hydroxy-4-cholesten-3-one if bile acid malabsorption is suspected 1

For Bloody or Inflammatory Diarrhea

• Stool for occult blood 1
• Fecal lactoferrin or calprotectin to detect inflammatory processes 1
• Culture for STEC with specific testing for Shiga toxin 1
• Monitor hemoglobin and platelet counts if STEC is suspected to detect early signs of hemolytic uremic syndrome 1

For Fatty Diarrhea

• Fecal elastase to assess for pancreatic insufficiency 1
• Qualitative fecal fat if malabsorption is suspected 1, 4

Special Considerations

• For travelers with diarrhea lasting 14 days or longer, specific testing for intestinal parasites is recommended 1
• In immunocompromised patients, additional testing should include Cryptosporidium, Cyclospora, Cystoisospora, microsporidia, Mycobacterium avium complex, and cytomegalovirus 1
• If C. difficile infection is suspected, check for leukocytosis which can be a harbinger and surrogate marker (mean WBC 15,800/mm³ in positive cases vs. 7,700/mm³ in negative cases) 5

When to Consider Endoscopic Evaluation

• Persistent diarrhea with negative initial laboratory workup 1
• Age >45 years (colonoscopy recommended due to increased risk of neoplasia) 1
• Presence of alarm features such as weight loss, nocturnal symptoms, or blood in stool 3
• For patients <45 years without alarm features, flexible sigmoidoscopy with biopsy may be sufficient 1

Follow-up Testing

• If initial testing is negative but symptoms persist, consider:
  • Testing for microscopic colitis (requires colonoscopy with biopsy) 2
  • Small bowel bacterial overgrowth (glucose hydrogen breath test) 1
  • Inflammatory bowel disease markers if not already checked 1
  • Non-infectious causes including irritable bowel syndrome and post-infectious IBS 1, 3

Common Pitfalls to Avoid

• Don't assume all chronic diarrhea is infectious; after 2 weeks, non-infectious causes become more likely 4, 3
• Don't forget to test for C. difficile even if the patient has not been hospitalized but has received antibiotics 1, 5
• Multiple-pathogen nucleic acid amplification tests detect DNA, not necessarily viable organisms, so clinical correlation is essential 1
• Remember that some patients may have more than one type of diarrhea simultaneously (e.g., both inflammatory and malabsorptive components) 2