Stool Testing for Intermittent Diarrhea
For a patient with intermittent chronic diarrhea, order: complete blood count, fecal calprotectin (or lactoferrin), IgA tissue transglutaminase with total IgA, and Giardia antigen testing as your initial stool workup. 1
Initial Blood Work
Start with screening blood tests that have high specificity for organic disease 1:
- Complete blood count - screens for anemia, infection, or inflammatory markers 1
- C-reactive protein and ESR - though not recommended specifically for IBD screening, useful for detecting inflammation 1
- Comprehensive metabolic panel - assesses electrolytes, renal function, and albumin 1
- Thyroid function tests - hyperthyroidism can cause chronic diarrhea 1
- IgA tissue transglutaminase (IgA-tTG) with total IgA - celiac disease screening with >90% sensitivity and specificity 1
If IgA is deficient, add IgG tissue transglutaminase or IgG deaminated gliadin peptides to detect celiac disease 1.
Stool Studies
Recommended Tests
Fecal calprotectin or fecal lactoferrin - screens for inflammatory bowel disease with reasonable accuracy 1. This is preferred over serum inflammatory markers for IBD screening 1.
Giardia antigen test or PCR - strongly recommended as Giardia is a common, treatable cause of chronic watery diarrhea with >95% sensitivity and specificity 1.
Fecal occult blood - basic screening test recommended in the evaluation 1.
Tests to AVOID in Most Cases
Do NOT routinely order ova and parasites (O&P) testing unless the patient has specific risk factors 1, 2:
- Recent travel to or immigration from high-risk endemic areas 1
- HIV-positive status 2
- Institutionalization 2
- Prior parasitic disease 2
The yield of O&P testing in patients without these risk factors is only 2.15%, making it cost-ineffective 2.
Stool culture for routine bacterial pathogens is generally low-yield in chronic diarrhea (>4 weeks duration) unless 1:
- Recent antibiotic use (test for C. difficile) 1
- Fever, bloody stools, or systemic symptoms suggesting acute infectious colitis 1
- Outbreak setting 1
Additional Considerations Based on Clinical Context
If Diarrhea-Predominant Pattern
Consider testing for bile acid diarrhea, though tests are limited in availability 1:
- 48-hour stool bile acid collection (if available) 1
- Serum fibroblast growth factor 19 (if available) 1
- Empiric trial of bile acid sequestrants is reasonable when testing unavailable 1
If Alarm Features Present
Alarm features warrant more extensive evaluation 1:
- Unexplained weight loss
- Blood in stool
- Nocturnal diarrhea
- Fever
- Age >50 years (higher colon cancer risk) 1
In these cases, proceed directly to colonoscopy with biopsies to evaluate for microscopic colitis, inflammatory bowel disease, or malignancy 1.
Common Pitfalls
Don't over-test initially - the history often provides more diagnostic value than extensive stool testing 1, 3. Medications, diet (lactose, caffeine, sorbitol), and prior surgery are common causes identifiable by history alone 1.
Avoid laxative screens early unless factitious diarrhea is suspected based on clinical context, as this becomes more relevant in specialist referral settings 1.
Remember functional disorders - irritable bowel syndrome is extremely common and can be diagnosed clinically with Rome criteria when initial testing is negative 1, 3.