How Should a Gastroenterologist Approach Chronic Diarrhea?

Chronic diarrhea is not a single diagnostic problem. It is a symptom with a wide differential diagnosis, and the most efficient evaluation begins by defining the phenotype before ordering tests. In an adult with diarrhea lasting four weeks or longer, the initial task is to identify alarm features, determine whether the stool pattern is watery, fatty or inflammatory, review medications and exposures, and decide whether the patient belongs in a focused “functional/watery diarrhea” pathway or needs broader investigation. A small number of high-yield tests can then separate common treatable diseases from disorders that require endoscopy, imaging or specialist testing.

Why classification matters

A useful first distinction is between watery diarrhea, fatty or malabsorptive diarrhea, and inflammatory diarrhea. Watery diarrhea may be secretory, osmotic or functional. Fatty diarrhea suggests maldigestion or malabsorption, including pancreatic exocrine insufficiency, celiac disease or extensive small-bowel disease. Inflammatory diarrhea is suggested by blood, urgency, nocturnal symptoms, fever, elevated inflammatory markers, hypoalbuminemia or objective evidence of intestinal inflammation.

This classification is not perfect, but it prevents a common error: applying an irritable bowel syndrome work-up to a patient whose history already suggests organic disease. The American Gastroenterological Association (AGA) guideline on laboratory evaluation of chronic watery diarrhea specifically excludes patients with alarm features such as weight loss, anemia, hypoalbuminemia, bloody diarrhea, obvious fat malabsorption, a relevant family history, or high-risk travel. Those patients require a broader evaluation rather than a minimalist laboratory panel.

Start with history before testing

The medication list is often diagnostic. Metformin, magnesium-containing products, laxatives, antibiotics, colchicine, proton pump inhibitors, nonsteroidal anti-inflammatory drugs, selective serotonin reuptake inhibitors, olmesartan and many other agents can contribute to diarrhea directly or through drug-associated enteropathy or microscopic colitis. Ask specifically about non-prescription supplements, sugar alcohols, excessive caffeine, alcohol, recent antibiotics and new drugs; these exposures are often omitted from a routine medication history.

The clinical history should define duration, stool frequency and consistency, nocturnal symptoms, urgency, incontinence, blood, weight loss, pain, fever and the relationship to meals or fasting. Previous intestinal resection, cholecystectomy, pancreatitis, pelvic radiotherapy, immune suppression and travel materially change the differential diagnosis. Family history of inflammatory bowel disease (IBD), celiac disease or colorectal cancer should lower the threshold for endoscopic investigation.

The physical examination is usually not diagnostic by itself, but it may reveal volume depletion, malnutrition, thyroid disease, peripheral signs of chronic liver disease, abdominal masses, perianal Crohn disease or systemic illness.

What should be tested first?

For an immunocompetent adult with chronic watery diarrhea and no alarm features, the best-supported initial tests are targeted rather than exhaustive. The AGA recommends testing for Giardia and celiac disease, suggests fecal calprotectin or lactoferrin to screen for intestinal inflammation, and suggests evaluation for bile acid diarrhea. Routine ova-and-parasite testing is not recommended in the absence of relevant travel or immigration risk, apart from Giardia testing. The guideline also recommends against using erythrocyte sedimentation rate or C-reactive protein alone to screen for IBD in this narrow low-risk population.

In routine gastroenterology practice, many patients do not fit that narrow definition. A reasonable basic evaluation commonly includes a complete blood count, electrolytes, renal function, liver chemistries, albumin, thyroid testing when clinically indicated, celiac serology and an inflammatory stool marker. Additional tests should be selected from the phenotype rather than added automatically.

Celiac disease remains an important, treatable cause of chronic diarrhea. Testing should generally begin with immunoglobulin A (IgA) tissue transglutaminase together with assessment for IgA deficiency. A positive or strongly suspicious result usually leads to upper endoscopy with duodenal biopsies while the patient is still consuming gluten. A negative test does not end the evaluation when pre-test probability is high, particularly in IgA deficiency or when the patient has already restricted gluten.

Do not miss bile acid diarrhea

Bile acid diarrhea is common, under-recognized and treatable. It may occur after terminal ileal disease or resection, after cholecystectomy, or without an obvious structural cause. Where validated testing is available, a positive test is preferable to repeated empiric treatment trials. Depending on country and laboratory access, testing may include selenium-75-homocholic acid taurine retention, fasting serum 7alpha-hydroxy-4-cholesten-3-one, or fecal bile acid measurement. When testing is unavailable, a carefully monitored therapeutic trial may be used, but an apparent response is not perfectly specific.

When should colonoscopy be performed?

Colonoscopy is appropriate when alarm features are present, colorectal pathology is possible, inflammatory markers are abnormal, symptoms persist despite a focused initial evaluation, or the patient is in an age/risk group that requires colorectal assessment for other reasons. Importantly, normal-appearing colonic mucosa does not exclude microscopic colitis. In a patient with chronic watery diarrhea, biopsies from the right and left colon should be obtained when microscopic colitis is in the differential diagnosis.

Microscopic colitis deserves particular attention in older adults and in patients exposed to medications associated with the disease. The typical presentation is chronic, non-bloody watery diarrhea, often with urgency or nocturnal stools. Diagnosis is histologic; the colon can look normal endoscopically.

When should the small bowel or pancreas be investigated?

Small-bowel imaging, capsule endoscopy and enteroscopy are not first-line tests for every patient. They become useful when symptoms, biomarkers or previous testing suggest Crohn disease, small-bowel bleeding, structural disease or another small-intestinal process. Magnetic resonance enterography avoids ionizing radiation and is particularly useful when Crohn disease is suspected.

Pancreatic exocrine insufficiency should be considered when there is steatorrhea, weight loss, pancreatic disease, pancreatic surgery or other compatible risk factors. Fecal elastase is commonly used as a non-invasive initial test, although watery stool can dilute the sample and produce a falsely low result. Cross-sectional imaging and direct pancreatic assessment may be required when clinical suspicion remains high.

What about small intestinal bacterial overgrowth?

Small intestinal bacterial overgrowth (SIBO) is frequently invoked in patients with bloating and diarrhea, but it should not become a default diagnosis for unexplained symptoms. The pre-test probability is higher in patients with altered anatomy, severe dysmotility, blind loops or other established risk factors. Breath tests have limitations, and repeated empiric antibiotic courses can create diagnostic confusion. The better sequence is to identify why SIBO would be biologically plausible in that patient and then test or treat accordingly.

A practical stepwise algorithm

For most adults, a disciplined sequence works better than a “shotgun” panel:

A practical algorithm

• Confirm that diarrhea is chronic (at least four weeks) and characterize stool type, frequency and timing.
• Identify alarm features: bleeding, anemia, weight loss, hypoalbuminemia, fever, nocturnal symptoms, significant family history or marked inflammatory features.
• Review prescription drugs, supplements, diet, alcohol, travel, surgery and immune status.
• Use targeted first-line testing: celiac serology, Giardia testing, fecal calprotectin or lactoferrin when appropriate, and basic blood tests guided by context.
• Consider bile acid diarrhea early rather than only after years of negative testing.
• Use colonoscopy with biopsies when alarm features, persistent symptoms or microscopic colitis are concerns.
• Escalate to small-bowel imaging, capsule endoscopy, pancreatic testing or other specialized studies only when the phenotype supports them.

Common errors to avoid

The first is equating chronic diarrhea with IBS-D before excluding a short list of treatable diseases. The second is ordering broad infectious panels repeatedly in low-risk patients. The third is failing to biopsy a normal-looking colon when microscopic colitis is plausible. The fourth is overlooking medications and bile acid diarrhea. The fifth is continuing to test without revisiting the original phenotype when the first round of studies is unrevealing.

What should trainees remember?

The goal is not to order every available test. It is to move from phenotype to probability. A patient with watery diarrhea and no alarm features needs a different pathway from a patient with weight loss, iron deficiency and nocturnal stools. High-quality evaluation is therefore less about the number of investigations than about the sequence in which they are chosen.

For the practising gastroenterologist, the most important contemporary shift is toward high-value testing: fecal inflammatory markers rather than nonspecific blood markers for low-risk watery diarrhea; routine attention to celiac disease, Giardia and bile acid diarrhea; and targeted biopsies and imaging when the clinical pattern warrants them. The result is a shorter path to diagnosis and less incidental testing.

Free further reading from Dr. Alan B. R. Thomson

For a broader diagnostic framework, see Dr. Thomson's Practice Review in Gastroenterology and First Principles of Gastroenterology and Hepatology in Adults & Children, both available as free digital downloads on GIandHepatology.com. These books provide the foundational clinical reasoning; the current article updates the diagnostic pathway against contemporary society guidance.

Frequently asked questions

What counts as chronic diarrhea?

Diarrhea persisting for four weeks or longer is generally considered chronic.

Which tests are most useful in chronic watery diarrhea?

In an otherwise low-risk adult, high-yield tests include celiac serology, Giardia testing, fecal calprotectin or lactoferrin, and evaluation for bile acid diarrhea.

Can colonoscopy be normal in microscopic colitis?

Yes. The mucosa often appears normal, so diagnosis depends on colonic biopsies.

When should chronic diarrhea prompt urgent or broader investigation?

Bleeding, anemia, weight loss, hypoalbuminemia, fever, marked nocturnal symptoms, significant family history or objective inflammation should broaden the evaluation.

References

1. Thomson ABR. Practice Review in Gastroenterology. CAPstone Academic Publishers; 2014. ISBN 978-1500855321.

2. Thomson ABR. First Principles of Gastroenterology and Hepatology in Adults and Children. 7th ed. Vols I-II. CAPstone Academic Publishers; 2013. ISBN 978-1494345624 and 978-1494345501.

3. Smalley W, Falck-Ytter C, Carrasco-Labra A, et al. AGA Clinical Practice Guidelines on the Laboratory Evaluation of Functional Diarrhea and Diarrhea-Predominant Irritable Bowel Syndrome in Adults. Gastroenterology. 2019;157(3):851-854. doi:10.1053/j.gastro.2019.07.004.

4. Arasaradnam RP, Brown S, Forbes A, et al. Guidelines for the investigation of chronic diarrhoea in adults: British Society of Gastroenterology, 3rd edition. Gut. 2018;67(8):1380-1399. doi:10.1136/gutjnl-2017-315909.