Chronic Diarrhea and Malabsorption

• Audio chapter
  Single-voice audio, listen on the commute.
• ABIM-format MCQs
  5-option vignettes with full wrong-answer teaching.
• Study guide
  Tables, decision trees, primary sources.
• AI tutor
  Chapter-grounded, answers the question you're stuck on.

What this chapter covers

• Section 12.1: Pathophysiologic classification

  Chronic diarrhea is a problem of too much stool water.

• Section 12.2: Initial evaluation and stool studies

  Chronic diarrhea evaluation begins with a layered workup that does cheap noninvasive things first and reserves invasive testing for the patients who need it.

• Section 12.3: Secretory diarrhea workup

  Secretory diarrhea is the bucket where the calculated osmotic gap is under 50 mOsm/kg, the volume runs high (often over 1 liter per day), the diarrhea persists with fasting, and nocturnal stools are common.

• Section 12.4: Fatty and inflammatory diarrhea

  Fatty diarrhea is the steatorrhea phenotype: greasy, foul-smelling, oil-floating stools with weight loss, fat-soluble vitamin (ADEK) deficiency, and the laboratory findings of hypocalcemia, low 25-hydroxyvitamin D, prolonged INR from vitamin K malabsorption, and night blindness from vitamin A deficiency.

• Section 12.5: Bile acid malabsorption framework

  Bile acid malabsorption is the syndrome in which excess bile acids enter the colon and stimulate water and electrolyte secretion through the bile-acid-sensitive G-protein-coupled receptor TGR5 (also called GPBAR1) on the colonic enterocyte.

• Section 12.6: Microscopic colitis

  Microscopic colitis is the umbrella for lymphocytic colitis and collagenous colitis, two histologic entities that produce chronic watery non-bloody diarrhea, normal-appearing colonic mucosa at colonoscopy, and a diagnosis that lives entirely in the biopsy.

• Section 12.7: Algorithm for obscure chronic diarrhea

  Obscure chronic diarrhea is the patient who has gone through the standard workup (history, medications, surgical history, basic labs with celiac serology and total IgA, calprotectin and elastase, infectious panel, stool osmolality and electrolytes, quantitative fecal fat, EGD with duodenal biopsies, colonoscopy with random biopsies including microscopic colitis sampling, and CT or MR enterography) without a diagnosis.

Podcast episodes

1. 01

   Framework and Mechanism Workup

   Chronic diarrhea is too much stool water, and because normal absorptive efficiency runs near ninety-nine percent, small physiologic insults produce large clinical phenotypes. Episode one builds the four-mechanism framework, osmotic, secretory, fatty, and inflammatory, and lets that mechanism drive a staged workup. History, basic labs, stool studies, and endoscopy sort most patients within a few tests toward a targeted second-stage evaluation.

   Read the transcript →
2. 02

   BAM Microscopic Obscure

   Episode two of the Chronic Diarrhea and Malabsorption chapter works the patient who arrives after a clean standard workup. Three entities account for most of what the first pass missed: bile acid malabsorption, microscopic colitis, and a structured algorithm for obscure chronic diarrhea. Board-tested reasoning on typing, testing, and the trials that are diagnostic and therapeutic in one step.

   Read the transcript →

Key topics

• ·Four mechanism categories of chronic diarrhea
• ·Stool osmotic gap calculation and cutoffs
• ·History, medication, and surgical review
• ·Basic labs and celiac serology with total IgA
• ·Stool studies: calprotectin, elastase, fecal fat
• ·IBS-D overlap traps
• ·Secretory workup and neuroendocrine tumors
• ·Fatty diarrhea: maldigestion versus malabsorption
• ·Bile acid malabsorption
• ·Camilleri classification
• ·Hundred-centimeter ileal resection rule
• ·Post-cholecystectomy diarrhea
• ·Microscopic colitis
• ·Lymphocytic vs collagenous colitis
• ·Obscure chronic diarrhea algorithm
• ·IBS-D overlap