Chronic Diarrhea and Malabsorption: Framework and Mechanism Workup

0:00Welcome to Board Pearls. This is episode one of two of the Chronic Diarrhea and Malabsorption chapter, in the Stomach and Small Bowel Disorders module. This episode is the framework and the mechanism-driven workup: sorting chronic diarrhea into osmotic, secretory, fatty, and inflammatory, and letting that mechanism drive the tests.

0:20Chronic diarrhea is fundamentally a problem of too much stool water. The gut handles nine or ten liters a day, the small intestine absorbs most of it, and the colon mops up nearly all of the rest, leaving a small amount in the stool. The reason that arithmetic matters is that absorptive efficiency normally runs about ninety-nine percent, so a drop to ninety-eight percent shows up clinically as diarrhea. That's why small physiologic insults produce a big clinical phenotype, and why the workup has to sort on mechanism rather than on the size of the insult. Chronic means loose stools for more than four weeks, and that cutoff isn't arbitrary: acute diarrhea is mostly infection that self-resolves, so once symptoms cross four weeks the differential pivots toward structural, inflammatory, malabsorptive, and endocrine causes that need a workup. So the first move on hearing chronic diarrhea is to reset away from infection toward the mechanism categories.

1:15There are four mechanism categories, and they exist because the workup that follows each is different. Osmotic diarrhea is the lumen holding water around poorly absorbed solutes, so removing the solute by fasting stops it, and the signature is a high stool osmotic gap that resolves with fasting. Secretory diarrhea is the bowel failing to absorb fluid, sometimes with active secretion on top, so the stool water is mostly ordinary electrolytes, the osmotic gap is low, and fasting doesn't stop it because the drive doesn't depend on what's in the lumen. Inflammatory diarrhea is bloody, leukocyte-positive, often febrile, and colonoscopy-driven because the answer usually lives in the colon. And fatty diarrhea is malabsorption or maldigestion of fat, whose workup splits along pancreatic versus mucosal lines.

2:06The osmotic gap is the calculation that mechanizes the sorting, and it should be reasoned rather than memorized. Stool osmolality is fixed at the serum value, about two hundred ninety, because water equilibrates the lumen with serum, and the measured electrolytes are sodium and potassium, doubled to account for their anions. So the gap is two hundred ninety minus twice the sum of stool sodium and potassium. A gap under fifty means the stool is almost entirely electrolyte-driven, so the diarrhea is secretory; a gap over one hundred means unmeasured solutes are carrying the osmotic load, so it's osmotic; in between is mixed. Two artifacts: a measured stool osmolality far below two hundred ninety means water or urine contamination diluting the sample, and far above means bacterial fermentation in a stool left at room temperature.

2:59Osmotic diarrhea sits on a short, scrollable list. Carbohydrate malabsorption is the most common driver, lactose in the lactase-deficient patient, excess fructose, the sorbitol and mannitol in sugar-free gums, and undigested starch in pancreatic insufficiency, along with magnesium-containing antacids and laxatives and the other poorly absorbed ions. The signature on history is resolution with fasting, because the offending solute has to be ingested, and a stool pH under six supports carbohydrate fermentation, since colonic bacteria ferment the unabsorbed carbohydrate into acids. So scrolling the diet and medication list before sending fancy tests solves most of these at the bedside.

3:41Secretory diarrhea is the larger and more dangerous category, including the endocrine tumors, microscopic colitis, bile acid malabsorption, chronic infection, and idiopathic disease. The volume tends to be high, it persists with fasting, and nocturnal stools are common because there's no feeding rhythm to gate it. The mechanism is dysregulation of the second-messenger pathways that drive chloride and bicarbonate secretion and inhibit sodium absorption, and the bacterial enterotoxins of cholera, enterotoxigenic E. coli, and C. diff all act through those same messengers, which is why secretory diarrhea looks similar across very different causes. Inflammatory diarrhea is signaled by blood, leukocytes, fever, and pain, with stool showing leukocytes, lactoferrin, or elevated calprotectin, and the differential is inflammatory bowel disease, invasive infection, ischemic or radiation colitis, and microscopic colitis, which is the exception in this group because it's histologically inflammatory but clinically watery. Fatty diarrhea is greasy, foul-smelling stools with weight loss and fat-soluble vitamin deficiency, splitting at maldigestion, which fails the luminal phase, meaning pancreatic insufficiency or inadequate bile, versus malabsorption, which fails the mucosal phase, meaning celiac, tropical sprue, Whipple, and the rest. And mixed pictures are common, so the four categories organize the workup rather than forcing a single label.

5:12That framework is the backbone of the rest of the episode, because the workup is built to assign the patient to one of those four mechanisms so the targeted second-stage tests can follow. The evaluation begins with the history, because that's where the diagnosis is in most patients, usually in the medication list, the surgical history, the travel history, or a diet diary nobody's asked about. Two features pull hard away from IBS and toward organic disease: nocturnal stools, because functional bowel disease respects the day-night cycle, and weight loss, because functional disease rarely causes it. The medication list is the single most productive part, including the over-the-counter and supplement list, magnesium products, sorbitol-sweetened candies, and laxatives, plus prescription offenders like metformin, antibiotics, acid blockers, SSRIs, and many others, with olmesartan deserving its own mention for the celiac-mimicking enteropathy. Surgical history is the next high-yield slice, and each operation maps to a mechanism: cholecystectomy and ileal resection to bile acid diarrhea, gastrectomy and bypass to dumping, and pancreatic resection to exocrine insufficiency, so learning the surgical list prepares you for the post-surgical vignettes. The exam targets dehydration and malnutrition plus the syndromic findings, each pointing somewhere specific, flushing with a right-heart murmur to carcinoid, migratory necrolytic erythema to glucagonoma, dermatitis herpetiformis to celiac, and the rhythmic eye-and-jaw movements to Whipple, and a rectal exam is mandatory because fecal incontinence is misread as diarrhea more often than expected, and its treatment is biofeedback, not another stool study.

6:53The basic labs are a blood count, metabolic panel, C-reactive protein, thyroid function, and the celiac serology with a total IgA. The blood count catches the anemia patterns, iron deficiency suggesting celiac, IBD, or malignancy, and macrocytic anemia suggesting B12 or folate loss from ileal disease, tropical sprue, or overgrowth. The metabolic panel catches the low potassium and acidosis of high-output secretory diarrhea and the low albumin of protein loss, and thyroid function is included because hyperthyroidism speeds motility. And the total IgA goes with the celiac serology because IgA deficiency invalidates the IgA-based test, so forgetting it is the single most common workup error in the celiac-overlap vignette.

7:44Stool studies are the second layer and do the mechanism assignment. Fecal calprotectin discriminates inflammatory from non-inflammatory diarrhea, with a low value essentially excluding IBD, an indeterminate middle zone often warranting a repeat, and a high value supporting inflammation and triggering colonoscopy, which makes it the favored way to sort a patient toward IBS or IBD without immediately scoping. Fecal elastase measures pancreatic function in a single random stool, with a low value supporting insufficiency and a very low value meaning severe, though a watery sample falsely lowers it, so you collect a formed sample. Stool osmolality and electrolytes give the osmotic gap, a Sudan stain is a qualitative fat screen, and a seventy-two-hour quantitative fecal fat on a controlled fat diet is the gold standard. The infectious panel is multiplex PCR with a separate C. diff toxin test after antibiotics or in a hospitalized patient.

8:41Imaging and endoscopy are the third layer. CT or MR enterography evaluates structural small-bowel disease, and colonoscopy with ileoscopy and biopsies is the move when the diarrhea is bloody, when calprotectin is up, or when the secretory workup needs to exclude microscopic colitis. And random biopsies of normal-looking colon are mandatory in any older patient with chronic watery diarrhea, because microscopic colitis is invisible to the eye and lives entirely in the histology. Endoscopy with the celiac biopsy protocol is the move when steatorrhea or mucosal disease is suspected.

9:20The IBS-D overlap deserves explicit mention, because it's where the boards trap candidates: among patients meeting IBS-D criteria, a large share have food intolerance, many have bile acid malabsorption, a meaningful fraction have bacterial overgrowth, and smaller groups have microscopic colitis or celiac. That means most patients carrying an IBS-D label actually have a specific condition with targeted therapy, so accepting the label without running the basic workup and considering the bile acid, overgrowth, and microscopic colitis trio is the punished move. The patient with bloating and diarrhea triggered by bread, apples, and garlic with negative IgE panels isn't allergic to anything, she's intolerant of fructans and polyols, and the next move is an elimination diet, not another allergy panel.

10:06Once the mechanism is assigned, the secondary workup follows. In the secretory category, where the gap is under fifty, the volume is high, and it persists with fasting, the bedside confirmation is a supervised fast: maintained volume points to a non-luminal driver like a toxin, a neuroendocrine tumor, bile acid malabsorption, microscopic colitis, or idiopathic disease, while a drop points to a missed osmotic component. The differential runs common to rare: microscopic colitis is statistically the leading cause of chronic watery secretory diarrhea in older adults and is episode two, bile acid malabsorption is the next high-yield cause and the leading cause of unexplained secretory diarrhea after cholecystectomy or ileal resection, where an empiric cholestyramine trial is often diagnostic and therapeutic in one step, and drug and infectious causes get screened off the medication list and the PCR panel.

11:04The neuroendocrine tumors are rare but tested, and the rule is to recognize the syndrome before reflexively ordering the panel, so dramatic volume, electrolyte derangement, or a syndromic finding earns the workup. The screening panel is chromogranin A, VIP, gastrin off the acid blocker for a week, calcitonin, somatostatin, glucagon, and a urine 5-HIAA, with the caveat that acid blockers raise both chromogranin A and gastrin and cause false positives, which is why they're held for a week. VIPoma is the prototype secretory tumor, the pancreatic cholera syndrome, and the acronym is watery diarrhea, hypokalemia, achlorhydria: VIP raises the enterocyte second messenger to drive chloride and bicarbonate secretion and suppress sodium absorption, and it suppresses gastric acid, giving the achlorhydria, with flushing from its vasodilation. Most VIPomas are pancreatic and large by diagnosis, often with liver metastases, and the diagnosis is a high fasting VIP with large-volume secretory diarrhea plus imaging, treated by resection if localized and somatostatin analogs and radionuclide therapy in advanced disease.

12:16Gastrinoma presents with severe acid-peptic disease plus diarrhea, which is Zollinger-Ellison, and the diarrhea is mixed because the massive acid output overwhelms pancreatic bicarbonate, denatures lipase, and damages the mucosa, giving both secretory and steatorrhea components on top of the ulcers. Most gastrinomas are duodenal and sit in the gastrinoma triangle, and about a quarter go with MEN1, so a diagnosis triggers parathyroid and pituitary screening. Diagnosis is a fasting gastrin off the acid blocker with a gastric pH: a very high gastrin with a low pH establishes it, an intermediate gastrin with low pH goes to a secretin test looking for the paradoxical rise, and a normal gastrin essentially excludes it. Treatment is high-dose acid suppression plus resection when feasible. Carcinoid syndrome occurs almost only when a midgut tumor has metastasized to the liver, bypassing first-pass clearance to dump serotonin and other mediators systemically, giving the triad of flushing, secretory diarrhea, and wheezing, with right-sided valve disease over time, diagnosed by urine 5-HIAA and treated with somatostatin analogs plus telotristat for refractory diarrhea. And the other endocrine causes each have a giveaway: medullary thyroid cancer with calcitonin and a MEN2 history, glucagonoma with its dermatitis, diabetes, clots, and depression, somatostatinoma with steatorrhea, stones, and sugar, mastocytosis with flushing and urticaria pigmentosa, and Addison with hyperpigmentation and the electrolyte pattern. And don't forget the factitious causes the boards always probe: surreptitious laxative use, classically a young woman with a normal organic workup, diagnosed with laxative screens and melanosis coli on colonoscopy.

14:01So the order of operations for secretory diarrhea: fast the patient to confirm the mechanism, run the cholestyramine trial because bile acid malabsorption is common and the trial is therapeutic in the same step, scope with random biopsies to catch microscopic colitis, and order the neuroendocrine panel when features are dramatic or syndromic.

14:21Fatty diarrhea is the steatorrhea phenotype, with a lab signature of low calcium, low vitamin D, a prolonged INR from vitamin K loss, and night blindness from vitamin A loss, and the mechanism splits at maldigestion versus malabsorption. Quantitative fecal fat on a controlled fat diet is the gold standard, with normal under seven grams a day, the favored cutoff for calling steatorrhea being over fourteen with diarrhea, and severe steatorrhea over thirty being typical of pancreatic insufficiency while intermediate values are more common in mucosal disease. The maldigestion-versus-malabsorption split is made by fecal elastase, biopsy, and context: a low elastase supports pancreatic insufficiency, whose causes are chronic pancreatitis, pancreatic cancer, cystic fibrosis, and post-Whipple anatomy, treated with enzyme replacement dosed to lipase per meal and per snack taken with the first bite, plus fat-soluble vitamins. Mucosal causes are evaluated with duodenal biopsy and celiac serology, with celiac the highest-yield, its steatorrhea coming from lost absorptive surface, cured with a gluten-free diet. The other mucosal causes round out the list, tropical sprue, Whipple, autoimmune enteropathy, common variable immunodeficiency, lymphangiectasia with protein loss, short bowel, and bacterial overgrowth, which produces steatorrhea by deconjugating bile acids so they can't form micelles, diagnosed by breath testing and treated with rifaximin. And bile acid maldigestion itself produces steatorrhea when the luminal bile acid concentration falls too low, in severe cholestasis, in overgrowth, and in massive ileal resection, treated with the underlying cause plus medium-chain triglycerides, which are absorbed without micelles.

16:15Inflammatory diarrhea is the last category and the workup is colonoscopy-driven, with stool showing blood, leukocytes, lactoferrin, and a high calprotectin. The colonoscopy-priority differential is inflammatory bowel disease, invasive infection, C. diff, CMV in the immunocompromised, ischemic and radiation colitis, and colorectal cancer. And the boards reward matching the endoscopic and biopsy pattern to the cause: pseudomembranes with a positive toxin for C. diff, skip lesions and granulomas for Crohn's, continuous distal inflammation with crypt distortion and basal plasma cells for ulcerative colitis, watershed necrosis at the splenic flexure for ischemia, telangiectasias in a radiation field for radiation colitis, owl-eye inclusions for CMV, and normal mucosa with either intraepithelial lymphocytosis or a thick collagen band for microscopic colitis, the exception that's watery rather than bloody, covered in episode two.

17:10So that's the framework and the mechanism-driven workup. The four categories are osmotic, secretory, fatty, and inflammatory. The osmotic gap does the first sort, under fifty pointing secretory and over one hundred pointing osmotic. The fast trial confirms secretory. The calprotectin sorts inflammatory from non-inflammatory. The fecal elastase sorts pancreatic from mucosal within the fatty category. The duodenal biopsy and celiac serology address the mucosal causes, and the colonoscopy with random biopsies catches the inflammatory and microscopic categories. Most chronic diarrhea sorts cleanly through that sequence within a few tests, and the targeted second-stage workup follows from the mechanism the first stage assigns.

17:55Episode two picks up the three entities the standard workup misses most often: bile acid malabsorption with its classification, its FGF19 mechanism, and the hundred-centimeter rule; microscopic colitis with its two subtypes, the right-colon biopsy requirement, and budesonide-first management; and the obscure chronic diarrhea algorithm for the patient still without an answer.

18:16For the full chapter, the practice vignettes, and the topic-tagged question bank, head to board pearls dot com. You'll find the rest of the series on Apple Podcasts, Spotify, or wherever you listen to podcasts. That brings us to the end of episode one of two of chapter twelve, and I'll see you in the next one.