Clinical meaning
Chronic diarrhea (≥ 4 weeks duration) is classified by mechanism: osmotic (unabsorbed solutes draw water into the lumen—lactose intolerance, sorbitol, celiac disease), secretory (active ion secretion exceeds absorption—VIPoma, bile acid malabsorption, microscopic colitis), inflammatory (mucosal damage with blood/pus—IBD, infection, ischemia), and motility-related (altered transit time—IBS, hyperthyroidism, diabetic autonomic neuropathy). The osmotic gap ([290 − 2 × (Na⁺ + K⁺)] in stool) helps differentiate: > 50 = osmotic, < 50 = secretory. A systematic algorithmic approach using stool studies, serologies, and endoscopy narrows the differential.
Diagnosis & workup
Diagnostics & workup: - Stool studies: culture, ova and parasites, C. difficile toxin, fecal calprotectin - Fecal fat (72-hour collection or Sudan stain) for steatorrhea - Stool osmotic gap calculation to differentiate osmotic vs secretory - Celiac serologies (tissue transglutaminase IgA with total IgA level) - TSH to rule out hyperthyroidism - Colonoscopy with random biopsies (even if mucosa appears normal—for microscopic colitis) - SeHCAT scan or empiric cholestyramine trial for bile acid diarrhea - Fecal elastase for pancreatic exocrine insufficiency