Diagnostic Tests for Diarrhea
Initial Stratification by Duration
The critical first step is determining whether diarrhea is acute (<4 weeks) or chronic (≥4 weeks), as this fundamentally changes your diagnostic approach. 1, 2
Acute Diarrhea (<4 weeks)
When to Test
Most acute diarrhea is self-limiting and does not require diagnostic workup. 3 Reserve testing for patients with:
- Fever 4, 3
- Bloody or mucoid stools 4, 3
- Severe abdominal cramping 1
- Signs of sepsis or severe dehydration 1, 3
- Immunocompromised status 1
- Suspected nosocomial infection 3
Stool Testing for Acute Diarrhea
- Bacterial culture: Salmonella, Shigella, Campylobacter, Yersinia 4, 1
- C. difficile toxin: Especially with antibiotic use within preceding 8-12 weeks 4, 1
- Shiga toxin-producing E. coli (STEC): Culture on sorbitol-MacConkey agar for O157, plus genomic assays for non-O157 strains 4, 1
- Ova and parasites: Giardia, Cryptosporidium, Cyclospora, Entamoeba histolytica 4
Blood Work for Acute Diarrhea
- Complete blood count with differential 4, 1
- Basic metabolic panel (electrolytes, renal function) 4
- Blood cultures if: infant <3 months, signs of septicemia, suspected enteric fever, or immunocompromised 1
Special Testing for Bloody Diarrhea
- Fecal occult blood 4
- Fecal lactoferrin or calprotectin to detect inflammation 4
- Monitor hemoglobin and platelets if STEC suspected (early detection of hemolytic uremic syndrome) 4
Chronic Diarrhea (≥4 weeks)
Red Flag Symptoms Requiring Urgent Evaluation
These mandate immediate gastroenterology referral and aggressive workup: 1
- Nocturnal diarrhea 1
- Unintentional weight loss 5, 1
- Blood in stool 5, 1
- Persistent fever 1
- Age >45 years with new-onset symptoms 1
First-Line Laboratory Tests
All patients with chronic diarrhea should receive: 1
- Complete blood count 5, 1
- Inflammatory markers: ESR, C-reactive protein 5, 1
- Comprehensive metabolic panel (electrolytes, renal function, liver function) 5, 1
- Celiac serology: Anti-tissue transglutaminase IgA with total IgA 5, 1
- Thyroid function tests 5, 1
- Nutritional markers: Iron studies, vitamin B12, folate, calcium 1
Stool Studies for Chronic Diarrhea
- Fecal calprotectin (screens for inflammatory bowel disease) 1
- Stool culture and microscopy 1
- Laxative screen (especially in specialist referral practice to detect factitious diarrhea) 5, 1
Categorization by Stool Type
Watery Diarrhea
- Stool electrolytes (Na+, K+) to calculate osmotic gap 4
- SeHCAT scan or serum 7α-hydroxy-4-cholesten-3-one for bile acid malabsorption 5, 4
- Fecal bile acid measurement where available 5
Fatty Diarrhea (Steatorrhea)
- Fecal elastase to assess pancreatic insufficiency 4
- Qualitative fecal fat if malabsorption suspected 4
Inflammatory/Bloody Diarrhea
- Fecal calprotectin or lactoferrin 4
- Colonoscopy with biopsies (mandatory—cannot diagnose microscopic colitis clinically) 1
Endoscopic Evaluation
Age-Stratified Approach
- Age ≥45 years: Full colonoscopy with biopsies (mandatory to exclude colorectal cancer) 1
- Age <45 years without alarm features and normal fecal calprotectin: Flexible sigmoidoscopy acceptable 1
- Persistent diarrhea with negative initial workup: Proceed to endoscopy 4
Special Populations
Immunocompromised Patients
Broader differential requires additional testing for: 4, 1
- Cryptosporidium, Cyclospora, Cystoisospora 4, 1
- Microsporidia 4, 1
- Mycobacterium avium complex 4, 1
- Cytomegalovirus 4, 1
Travelers with Persistent Diarrhea (≥14 days)
- Specific testing for intestinal parasites 4, 1
- C. difficile if antimicrobial treatment received within 8-12 weeks 1
Critical Pitfalls to Avoid
- Don't diagnose IBS prematurely: Complete basic blood and stool screening first 1
- Always test for celiac disease: Anti-tissue transglutaminase IgA with total IgA is mandatory 1
- Don't skip colonoscopy in older patients: Full colonoscopy is non-negotiable in patients ≥45 years 1
- Remember microscopic colitis: Requires colonoscopy with biopsies—cannot be diagnosed clinically 1
- Screen for laxative abuse early: Especially in specialist referral settings 5, 1
- Nucleic acid amplification tests detect DNA, not viable organisms: Clinical correlation is essential 4
- Don't forget C. difficile in outpatients: Test even without hospitalization if recent antibiotics 4