What lab tests are recommended for a patient presenting with diarrhea?

Laboratory Testing for Diarrhea

For patients presenting with diarrhea, the recommended laboratory tests include stool testing for Giardia, fecal calprotectin or lactoferrin to screen for inflammatory bowel disease, and celiac disease serologies. 1, 2

Initial Laboratory Evaluation

Basic Screening Tests

• Complete blood count (CBC)
• Erythrocyte sedimentation rate (ESR)
• C-reactive protein (CRP)
• Electrolytes
• Liver function tests
• Iron studies
• Vitamin B12 and folate
• Thyroid function tests 1, 2

These tests have high specificity but low sensitivity for organic disease 1.

Stool-Based Testing

1. Inflammatory Markers:

   • Fecal calprotectin or fecal lactoferrin (more sensitive than ESR or CRP for detecting intestinal inflammation) 1, 2

2. Pathogen Testing:

   • Giardia testing (strongly recommended regardless of travel history) 1, 2
   • Bacterial culture for Salmonella, Shigella, Campylobacter, and Yersinia in symptomatic hosts 1
   • C. difficile testing for patients with recent antibiotic use or healthcare-associated diarrhea 1, 2

3. Celiac Disease Testing:

   • IgA tissue transglutaminase (tTG)
   • Second test to detect celiac disease in IgA deficiency (IgG-tTG or IgG/IgA deaminated gliadin peptides) 1, 2

4. Fecal Occult Blood Test (Hemoccult) 2

Testing Based on Clinical Context

Chronic Diarrhea

• Bile acid diarrhea testing (48-hour stool collection for total bile acids or serum fibroblast growth factor 19) 1, 2
• If bile acid testing is unavailable, consider empiric trial of bile acid binders 1, 2

Travel-Related or High-Risk Situations

• Stool for ova and parasites (only recommended with travel history to or immigration from high-risk areas) 1
• Vibrio species testing for patients with rice water stools, exposure to brackish waters, consumption of raw shellfish, or travel to cholera-endemic regions 1

Immunocompromised Patients

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

Suspected Outbreak

• Testing for multiple bacterial, viral, and parasitic agents in coordination with public health authorities 1

When to Consider Endoscopy

• Colonoscopy with biopsies for:
  • Patients over 45-50 years
  • Persistent diarrhea with weight loss
  • Evaluation for microscopic colitis 1, 2

Important Considerations

Avoiding Unnecessary Testing

• Routine stool ova and parasite testing has very low yield (1.5-3%) in patients without risk factors 1, 3, 4
• For nosocomial diarrhea (>3 days after hospitalization), routine stool cultures and ova/parasite testing are not recommended; focus on C. difficile testing instead 1, 5

Factitious Diarrhea

• Consider screening for laxative abuse in cases of persistent unexplained diarrhea
• Tests should include detection of anthraquinones, bisacodyl, and phenolphthalein in urine, and magnesium and phosphate in stool 1

Testing Algorithm

1. Start with basic blood tests and stool tests for Giardia, inflammatory markers, and celiac disease
2. If symptoms persist >7 days, add targeted testing based on risk factors
3. For patients >45 years or with alarm features (weight loss, blood in stool), proceed to colonoscopy
4. For suspected bile acid diarrhea, consider specific testing or empiric trial of bile acid binders

By following this evidence-based approach to laboratory testing for diarrhea, clinicians can efficiently identify the underlying cause while avoiding unnecessary tests.