What are the initial laboratory tests and treatment approaches for patients presenting with diarrhea?

Initial Laboratory Tests and Treatment Approaches for Diarrhea

The initial laboratory assessment for patients presenting with diarrhea should include a screening blood test (full blood count, ferritin, tissue transglutaminase/EMA and thyroid function test) as well as stool tests for inflammation (faecal calprotectin). 1

Initial Assessment

Laboratory Testing

• Basic blood tests:

  • Complete blood count (to assess for anemia, leukocytosis suggesting infection)
  • Basic metabolic panel (to evaluate electrolyte disturbances and renal function)
  • Ferritin (to screen for iron deficiency)
  • Tissue transglutaminase/EMA (to screen for celiac disease)
  • Thyroid function tests (to rule out hyperthyroidism as a cause)

• Stool tests:

  • Fecal calprotectin (to screen for inflammatory bowel disease)
  • C. difficile testing (especially with recent antibiotic use or healthcare exposure)
  • Stool cultures for infectious pathogens when indicated

Indications for Stool Testing

Stool testing for infectious pathogens is indicated in patients with:

• Fever
• Bloody or mucoid stools
• Severe abdominal cramping/tenderness
• Signs of sepsis
• Recent antibiotic use (for C. difficile)
• Immunocompromised status
• Recent travel to endemic areas
• Suspected outbreak 2

Treatment Approach

Rehydration

• Oral rehydration is the cornerstone of treatment for most cases of diarrhea
• In large-volume, dehydrating diarrhea, oral rehydration solutions should be used as they are formulated to stimulate sodium and water absorption 3
• IV fluids may be necessary for severe dehydration

Antimotility Agents

• Loperamide can be useful in reducing the number of bowel movements and fluid loss
• Caution: Avoid in patients with fever, bloody diarrhea, or suspected inflammatory/invasive diarrhea 4
• Monitor for adverse effects including constipation (1.7% overall incidence) 4

Antibiotic Therapy

• Generally not recommended for most cases of acute diarrhea
• Consider empiric antibiotics in:
  • Patients with fever, abdominal pain, and bloody diarrhea
  • Severe travelers' diarrhea
  • Suspected enteric fever
  • Immunocompromised patients 2

Specialized Testing Based on Duration and Presentation

Acute Diarrhea (<4 weeks)

• Focus on infectious causes and dehydration assessment
• Most cases are self-limited and require minimal investigation

Chronic Diarrhea (>4 weeks)

• More extensive testing is warranted:
  • Giardia testing (antigen test or PCR)
  • Celiac disease serologies (IgA tTG and second tests for IgA deficiency)
  • Consider colonoscopy with biopsies for patients:
    • Over 45-50 years
    • With persistent diarrhea and weight loss
    • For evaluation of microscopic colitis 1, 2

Algorithm for Investigation

1. Initial assessment: Determine if acute (<4 weeks) or chronic (>4 weeks)
2. Screen for red flags: Fever, bloody stools, weight loss, severe abdominal pain
3. Basic laboratory testing: CBC, electrolytes, liver function, thyroid function, celiac serology
4. Stool studies: Based on clinical presentation (as outlined above)
5. Further investigation for persistent symptoms:
   • For chronic diarrhea: Consider colonoscopy with biopsies
   • For suspected malabsorption: Consider small bowel imaging or biopsy
   • For suspected bile acid diarrhea: Consider 48-hour stool collection for bile acids or empiric trial of bile acid binders 1, 2

Special Considerations

• Immunocompromised patients: Lower threshold for testing and broader diagnostic approach
• Recent antibiotic use: High suspicion for C. difficile infection
• Recent travel: Consider region-specific pathogens
• Age >45 years: Lower threshold for endoscopic evaluation to exclude neoplasia 1

By following this systematic approach to laboratory testing and treatment, clinicians can efficiently diagnose and manage patients presenting with diarrhea while minimizing unnecessary testing and optimizing outcomes.