Diagnostic Testing for Functional Diarrhea Without Alarm Features
For a patient with frequent stools but no abdominal pain or blood, start with basic screening blood tests and stool studies to exclude organic disease before accepting a diagnosis of functional diarrhea.
Initial Laboratory Screening
The following tests should be ordered as first-line investigations to exclude common organic causes:
Blood Tests
- Complete blood count (CBC) to assess for anemia or infection 1
- Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) to screen for inflammatory conditions 1
- Comprehensive metabolic panel including electrolytes, liver function tests, calcium, and albumin 1
- Thyroid function tests (TSH) to exclude hyperthyroidism as a cause of diarrhea 1
- Tissue transglutaminase IgA antibodies with total IgA level to screen for celiac disease, the most common small bowel enteropathy in Western populations 1
- Iron studies, vitamin B12, and folate to assess for malabsorption 1
Stool Studies
- Stool culture and microscopy for ova and parasites, though infectious causes are uncommon in chronic diarrhea in immunocompetent patients from developed countries 1
- Fecal occult blood testing to screen for bleeding 1
- Laxative screen if stool weight exceeds 200g daily, as factitious diarrhea becomes increasingly common in specialist practice 1
Age-Stratified Endoscopic Evaluation
The need for endoscopy depends critically on patient age and risk factors:
Patients Under 45 Years
- Flexible sigmoidoscopy with biopsies is the preferred initial investigation, as the diagnostic yield is not substantially different from colonoscopy in younger patients 1
- Rectal biopsies should be obtained to exclude microscopic colitis 1
- Full colonoscopy is reserved for those with severe diarrhea, weight loss, or other concerning features 1
Patients Over 45 Years
- Full colonoscopy is recommended due to higher pretest probability of colorectal cancer 1
- Biopsies should be taken even if the mucosa appears normal to exclude microscopic colitis 1
Additional Testing for Diarrhea-Predominant Symptoms
If initial screening is negative and diarrhea persists, consider:
- Lactose hydrogen breath test for patients consuming more than 0.5 pint (280 ml) of milk or equivalent dairy products daily, especially in racial groups with high lactose malabsorption incidence 1
- Glucose hydrogen breath test to evaluate for small bowel bacterial overgrowth, though sensitivity is limited 1
- SeHCAT testing (where available) or empirical trial of cholestyramine to assess for bile acid malabsorption, particularly if stool weight exceeds 200g daily 1
- Small bowel biopsies via upper endoscopy if celiac serology is negative but malabsorption is suspected 1
Critical Pitfalls to Avoid
Do not diagnose functional diarrhea without excluding organic disease first. The diagnosis of functional bowel disorders presumes the absence of structural or biochemical explanation for symptoms 1.
Do not over-investigate young patients with typical functional symptoms. Young patients (<45 years) with typical functional symptoms, no alarm features (fever, weight loss, blood in stools, anemia, abnormal physical findings), and normal examination can be given a working diagnosis after basic screening tests 1.
Do not order hormone testing routinely. Testing for vasoactive intestinal peptide, gastrin, or glucagon is recommended only in high-volume watery diarrhea when other causes have been excluded, as these tumors are extremely rare 1.
When to Stop Testing
If the patient is under 45 years old, has no alarm features, normal physical examination, and all screening tests are negative, a diagnosis of functional diarrhea can be made without further invasive testing 1. The incidence of new significant diagnoses once functional diarrhea is established is extremely low 1.