Diagnostic Approach for Functional Diarrhea

Initial Evaluation

The American Gastroenterological Association recommends starting with basic screening blood tests and stool studies to exclude organic disease before accepting a diagnosis of functional diarrhea, including complete blood count (CBC), erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), comprehensive metabolic panel, thyroid function tests (TSH), tissue transglutaminase IgA antibodies, and iron studies 1, 2
Stool culture and microscopy for ova and parasites should be considered, though infectious causes are uncommon in chronic diarrhea in immunocompetent patients from developed countries 1
Fecal occult blood testing should be performed to screen for bleeding 3, 4

For patients consuming more than 0.5 pint (280 ml) of milk or equivalent dairy products daily, a lactose hydrogen breath test should be considered, especially in racial groups with high lactose malabsorption incidence 6
A glucose hydrogen breath test can be used to evaluate for small bowel bacterial overgrowth, though sensitivity is limited 1, 2
SeHCAT testing or an empirical trial of cholestyramine can be used to assess for bile acid malabsorption, particularly if stool weight exceeds 200g daily 1, 2, 6

For 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, 2
For patients over 45 years, full colonoscopy is recommended due to higher pretest probability of colorectal cancer 1, 3, 4

The diagnosis of functional bowel disorders presumes the absence of structural or biochemical explanation for symptoms, and organic disease should be excluded before making a diagnosis of functional diarrhea 3, 4
A diagnosis of functional diarrhea can be made without further invasive testing if the patient is under 45 years old, has no alarm features, normal physical examination, and all screening tests are negative 3, 4, 6