# Chronic Diarrhea

Use a pattern-based evaluation to identify inflammatory, malabsorptive, infectious, medication-related, postoperative, bile acid, pancreatic, and functional causes while rapidly correcting volume, electrolyte, and nutritional consequences.

**Clinical question:** How should clinicians evaluate adults with diarrhea persisting longer than 4 weeks and direct testing toward treatable etiologies?

Updated: 2026-08-24T18:29:36.882810+00:00

## What matters in practice
- Define chronic diarrhea as predominantly loose stools persisting for more than 4 weeks; begin by identifying dehydration, hypokalemia, acid-base disturbance, acute kidney injury, weight loss, bleeding, or suspected inflammatory disease. [11][22][24]
- In suspected functional diarrhea or IBS-D without alarm features, prioritize celiac serology, Giardia testing, and fecal inflammatory markers rather than broad simultaneous testing. [4][11]
- Use fecal calprotectin or lactoferrin to screen for intestinal inflammation; an abnormal result should redirect evaluation toward inflammatory bowel disease, infection, ischemia, or other inflammatory colonic disease. [4][11][23]
- Colonoscopy with mucosal biopsies is required when chronic watery diarrhea raises concern for microscopic colitis because colonoscopic appearance may be normal. [4][10]
- Test for bile acid diarrhea when chronic watery diarrhea remains unexplained; available approaches include a 48-hour fecal bile acid assay or serum fibroblast growth factor 19 measurement. [11]
- Greasy stools, weight loss, or nutritional abnormalities should prompt evaluation for malabsorption, including celiac disease and pancreatic exocrine insufficiency with fecal elastase-1. [18][22][24]

## Stabilize complications before labeling diarrhea functional

Correct physiologic consequences and identify features that require expedited organic-disease evaluation.

Assess volume status, orthostasis, urine output, serum creatinine, potassium, bicarbonate, and other basic metabolic panel abnormalities at presentation when stool losses are clinically consequential. Chronic diarrhea can produce dehydration, hypokalemia, non-anion-gap metabolic acidosis, and prerenal or multifactorial acute kidney injury; these findings warrant fluid and electrolyte replacement while the diagnostic workup proceeds. [22][24]

Escalate beyond a functional-disorder pathway when there is marked weight loss, greasy stools, nutritional deficiency, bleeding, systemic illness, or evidence of intestinal inflammation. Structural, inflammatory, infectious, malignant, malabsorptive, pancreatic, endocrine, medication-related, postoperative, and gut-brain interaction disorders can overlap symptomatically; diagnostic categorization should be driven by stool phenotype and targeted clinical clues rather than a presumptive IBS-D diagnosis. [13][14][18][24]

Document onset as abrupt versus gradual, continuous versus intermittent, and relation to medications, toxic exposures, alcohol, prior surgery, infection, or altered bowel anatomy. These exposures separate common secondary causes—medication effects, laxatives, postoperative physiology, postinfectious disease, and malabsorption—from primary inflammatory or functional disorders. [12][13][14][18]
- Obtain CBC, C-reactive protein, basic metabolic panel, IgA tissue transglutaminase antibody, and total IgA as an initial laboratory framework when evaluating chronic diarrhea. [24]
- Review all prescription, over-the-counter, and supplement exposures, specifically including laxatives, antacids, and sugar alcohol intake when an osmotic or medication-associated phenotype is possible. [14][18]
- Treat a new or persistent diarrheal syndrome in a patient with prior colectomy, bariatric surgery, ileal disease, pancreatic disease, or recent abdominal surgery as potentially anatomy- or malabsorption-related rather than functional by default. [3][13][18]

*Initial branch points that determine the next diagnostic action. [13][14][18][24]*

| Finding | Highest-yield etiologic branch | Next action |
| --- | --- | --- |
| Volume depletion, hypokalemia, metabolic acidosis, or acute kidney injury | High-output diarrheal loss requiring concurrent stabilization | Check basic metabolic panel and renal function; replace fluids and electrolytes while pursuing cause. [22][24] |
| Greasy stool, marked weight loss, or nutritional abnormalities | Fatty diarrhea or malabsorption | Test for celiac disease and evaluate pancreatic exocrine function with fecal elastase-1. [18][22][24] |
| Elevated fecal inflammatory marker or inflammatory clinical features | IBD, infection, ischemia, microscopic colitis, or malignancy | Proceed to directed endoscopic and infectious evaluation rather than diagnosing IBS-D. [4][14][23] |
| Watery diarrhea with no alarm features | Functional diarrhea, IBS-D, Giardia, celiac disease, bile acid diarrhea, or microscopic colitis | Perform focused celiac, Giardia, and fecal inflammatory testing; test for bile acid diarrhea or biopsy when indicated. [4][11] |

## Use focused testing before broad stool panels

The initial testing sequence should discriminate inflammatory, celiac, infectious, and bile acid-mediated disease.

For adults with chronic watery diarrhea compatible with functional diarrhea or IBS-D and without alarm features, order IgA tissue transglutaminase antibody with total IgA, test for Giardia, and obtain a fecal inflammatory marker such as calprotectin or lactoferrin. If IgA deficiency is present, use IgG tissue transglutaminase and IgG or IgA deamidated gliadin peptide testing for celiac disease. [4][11]

Avoid an indiscriminate "kitchen sink" approach in the low-alarm IBS-D or functional-diarrhea phenotype. AGA-oriented guidance favors celiac disease testing, Giardia assessment, and stool inflammatory markers first; subsequent testing should be determined by the phenotype and initial results. [4][11]

A positive fecal calprotectin or lactoferrin changes the pathway from functional disease to inflammatory evaluation. Fecal calprotectin is a noninvasive marker of gastrointestinal inflammation, but it is not disease-specific; interpret an abnormal test in the context of IBD, infection, ischemic colitis, microscopic colitis, and colorectal neoplasia risk, then select endoscopy, biopsy, or pathogen testing accordingly. [14][23][24]

When the differential remains broad after history, examination, and baseline blood testing, classify stool as watery, fatty, or inflammatory. This practical classification directs subsequent testing and accommodates overlap, since individual disorders may generate more than one pattern. [24]
- Celiac branch: positive serology or fatty diarrhea with weight loss should prompt evaluation for celiac disease rather than empiric functional-disease therapy. [4][22][24]
- Infectious branch: test for Giardia in chronic watery diarrhea; use additional stool microbiology selectively when exposure history, immunocompromise, or clinical context raises concern for persistent infection. [4][12][14]
- Inflammatory branch: fecal calprotectin or lactoferrin supports intestinal inflammatory assessment; C-reactive protein is included in ACG-oriented evaluation of chronic diarrhea or IBS-D without alarm symptoms. [11][23][24]
- Bile acid branch: use a 48-hour fecal bile acid assay or serum fibroblast growth factor 19 level when bile acid diarrhea is suspected. [11]

### Interpreting fecal elastase-1

Order fecal elastase-1 when steatorrhea, weight loss, nutritional abnormalities, pancreatic disease, or a maldigestive phenotype suggests pancreatic exocrine insufficiency. Fecal elastase-1 is the most commonly used indirect pancreatic function test; a value below 200 micrograms/g is suggestive of exocrine pancreatic insufficiency. [5][18]
- Reserve invasive secretin-cholecystokinin testing and endoscopic pancreatic function testing for selected pancreatic functional assessment scenarios; they are not routine tests for exocrine pancreatic insufficiency diagnosis. [18]

*Focused tests for common chronic diarrhea branches. [4][5][11][18][24]*

| Clinical pattern | Test | Result interpretation and next step |
| --- | --- | --- |
| Suspected functional diarrhea or IBS-D | IgA tissue transglutaminase plus total IgA | Screen for celiac disease; if IgA deficient, use IgG tissue transglutaminase and deamidated gliadin peptide testing. [4] |
| Chronic watery diarrhea | Giardia testing | A positive result identifies a treatable chronic gastrointestinal infection. [4][11] |
| Possible inflammatory diarrhea | Fecal calprotectin or lactoferrin | Abnormal results support intestinal inflammatory evaluation and should not be attributed to IBS-D alone. [4][11][23] |
| Unexplained watery diarrhea | 48-hour fecal bile acid assay or serum fibroblast growth factor 19 | Supports a bile acid diarrhea pathway. [11] |
| Fatty stool or pancreatic-risk phenotype | Fecal elastase-1 | Below 200 micrograms/g is suggestive of exocrine pancreatic insufficiency. [5][18] |

## Separate functional, bile acid, microscopic colitis, and osmotic causes

Watery stool does not establish IBS-D; several treatable disorders mimic a functional syndrome.

Diagnose IBS-D or functional diarrhea only after targeted exclusion of celiac disease, Giardia infection, and intestinal inflammation in an appropriate low-alarm presentation. Organic disorders that can resemble IBS include celiac disease, IBD, colorectal cancer, chronic gastrointestinal infection, microscopic colitis, and primary bile acid diarrhea. [4][6][11]

Consider bile acid diarrhea in persistent unexplained watery diarrhea, including patients otherwise labeled IBS-D. Testing options cited in current diagnostic pathways include a 48-hour fecal bile acid assay and serum fibroblast growth factor 19 measurement; an empiric bile acid binder trial may be a reasonable concomitant or subsequent strategy when diagnostic testing is unavailable or not widely accessible. [4][11]

In older adults with chronic watery diarrhea, proceed to endoscopic evaluation with biopsy to exclude microscopic colitis even when the mucosa appears normal. Microscopic colitis is defined histologically in the setting of chronic watery diarrhea and can lack significant colonoscopic abnormalities, so normal visual inspection alone does not exclude it. [4][10]

Review osmotic contributors before escalating invasive testing: lactose or other carbohydrate malabsorption, sorbitol and other sugar alcohols, laxatives, and antacids are recognized causes of osmotic diarrhea. A carefully timed withdrawal or dietary exposure assessment is often more decision-relevant than broad biomarker testing when the history identifies a plausible agent. [14]
- Microscopic colitis: obtain colonic mucosal biopsies during endoscopy; do not rely on grossly normal colonoscopy. [4][10]
- Bile acid diarrhea: consider testing in unexplained watery diarrhea and in IBS-D-like presentations that remain symptomatic after initial targeted testing. [11][19]
- Functional disorders: retain as a diagnosis when targeted testing is unrevealing and no structural, inflammatory, infectious, or malabsorptive discriminator emerges. [4][17][24]
- Medication and diet: ask specifically about laxatives, antacids, and sugar alcohol-containing products. [14][18]

*Clinical distinctions within chronic watery diarrhea. [4][6][10][11][14]*

| Branch | Discriminator | Action |
| --- | --- | --- |
| Functional diarrhea or IBS-D | Low-alarm phenotype after focused celiac, Giardia, and fecal inflammatory testing | Avoid broad simultaneous testing; direct subsequent evaluation by persistence and phenotype. [4] |
| Bile acid diarrhea | Persistent unexplained watery stool or IBS-D-like presentation | Obtain 48-hour fecal bile acid testing or serum fibroblast growth factor 19 where available; consider bile acid binder trial if testing is inaccessible. [4][11] |
| Microscopic colitis | Older patient with chronic watery diarrhea; colonoscopy may be macroscopically normal | Perform endoscopy with colonic biopsies. [4][10] |
| Osmotic diarrhea | Exposure to lactose or carbohydrate malabsorption triggers, laxatives, antacids, or sugar alcohols | Remove the suspected exposure and reassess stool response. [14] |

## Investigate malabsorption and inflammation with phenotype-directed testing

Fatty and inflammatory phenotypes warrant disease-specific testing and earlier endoscopic or pancreatic assessment.

Treat greasy stool and marked weight loss as a malabsorption signal. Celiac disease is a key cause of chronic fatty diarrhea and can produce laboratory evidence of malabsorption; initial testing uses tissue transglutaminase IgA with total IgA, with IgG-based serologies in IgA deficiency. [4][22][24]

Evaluate pancreatic exocrine insufficiency when diarrhea is accompanied by maldigestive symptoms, nutritional abnormalities, or pancreatic risk factors. Fecal elastase-1 is the commonly used indirect test, whereas direct secretin-cholecystokinin testing is invasive, costly, cumbersome, and generally reserved for selected functional assessment of chronic pancreatitis rather than routine exocrine insufficiency diagnosis. [18]

For inflammatory diarrhea, use fecal calprotectin or lactoferrin as a noninvasive screen and pursue endoscopic evaluation when clinical findings or biomarkers suggest intestinal inflammation. The differential includes Crohn disease, ulcerative colitis, infectious colitis, ischemic colitis, microscopic colitis, and colorectal malignancy. [14][18][23][24]

Use stool pathogen testing selectively rather than assuming chronic bacterial infection. Giardia is specifically prioritized in guideline-based functional-diarrhea and IBS-D evaluation, while broader stool culture, ova and parasite examination, and specialized testing are driven by clinical context and potential exposure to persistent pathogens. [4][12]
- Celiac disease: positive serology or strong malabsorptive suspicion redirects management toward confirmation and disease-specific treatment rather than symptom suppression. [4][22]
- Pancreatic exocrine insufficiency: fecal elastase-1 below 200 micrograms/g is suggestive and should be integrated with clinical and nutritional findings. [5][18]
- Inflammatory bowel disease or other colitis: abnormal stool inflammatory markers require evaluation for an organic inflammatory source. [4][11][23]
- Colorectal cancer: include in the organic differential when chronic diarrhea has alarm features or other concerning clinical findings. [6][14]

*Fatty and inflammatory phenotypes require different next tests. [4][5][14][18][22][24]*

| Phenotype | Leading considerations | Diagnostic next step |
| --- | --- | --- |
| Greasy stool with weight loss | Celiac disease; pancreatic exocrine insufficiency | Order celiac serologies and fecal elastase-1. [18][22][24] |
| Fecal elastase-1 below 200 micrograms/g | Pancreatic exocrine insufficiency | Interpret with nutritional markers, symptoms, and pancreatic evaluation. [5][18] |
| Inflammatory marker elevation | IBD, infection, ischemia, microscopic colitis, malignancy | Perform directed endoscopic and infectious evaluation. [14][23][24] |
| Chronic watery stool with normal-appearing colon | Microscopic colitis remains possible | Obtain colonic biopsies. [4][10] |

## Use endoscopy, biopsy, and specialized testing when initial branches remain unresolved

Escalation is indicated when targeted testing identifies inflammation, malabsorption, anatomic risk, or persistent unexplained symptoms.

Proceed to colonoscopy with biopsies when chronic diarrhea is accompanied by alarm features, an inflammatory phenotype, concern for colorectal cancer or IBD, or persistent watery diarrhea in an older adult where microscopic colitis is plausible. Biopsy is essential for microscopic colitis because diagnostic mucosal inflammation may occur without significant endoscopic abnormalities. [4][6][10][14]

Use specialized pancreatic testing sparingly. Direct secretin-cholecystokinin testing and endoscopic pancreatic function testing are invasive or not routinely available and are principally used when assessing chronic pancreatitis with inconclusive imaging, not as first-line tests for exocrine pancreatic insufficiency. [18]

Refer persistent unexplained cases for mechanism-focused evaluation rather than repeating the same low-yield screening tests. The next branch should be selected from the unresolved phenotype: bile acid diarrhea testing for watery stool, biopsy for microscopic colitis, pancreatic assessment for fatty stool, or endoscopic inflammatory evaluation for abnormal calprotectin or lactoferrin. [4][11][18][24]

Monitor the consequences of ongoing stool loss during the diagnostic interval. Recheck renal function, electrolytes, and acid-base status when initial testing shows volume depletion, potassium loss, metabolic acidosis, or kidney injury, because correction of these abnormalities is independent of the eventual etiologic diagnosis. [22][24]
- Do not exclude microscopic colitis after a visually normal colonoscopy unless mucosal biopsies were obtained. [4][10]
- Do not use a normal gross endoscopic appearance to settle a chronic watery-diarrhea evaluation in an older patient. [4][10]
- Do not use invasive pancreatic function tests as routine replacements for fecal elastase-1 in suspected exocrine pancreatic insufficiency. [18]
- Reassess the diagnosis if presumed IBS-D fails the initial focused workup pathway or develops weight loss, malabsorptive features, inflammatory markers, or other alarm findings. [4][6][17][24]

*Escalation decisions after the initial chronic diarrhea workup. [4][10][11][18][22][24]*

| Unresolved problem | Escalating test or procedure | Reason |
| --- | --- | --- |
| Older patient with persistent watery diarrhea | Colonoscopy with mucosal biopsies | Exclude microscopic colitis despite potentially normal endoscopic appearance. [4][10] |
| Elevated fecal calprotectin or lactoferrin | Directed endoscopic evaluation and pathogen assessment | Identify inflammatory, infectious, ischemic, or neoplastic disease. [14][23][24] |
| Persistent unexplained watery diarrhea | 48-hour fecal bile acid assay or serum fibroblast growth factor 19 | Evaluate bile acid diarrhea. [11] |
| Steatorrhea or suspected pancreatic maldigestion | Fecal elastase-1; selected pancreatic assessment if needed | Screen for exocrine pancreatic insufficiency before considering invasive testing. [5][18] |
| Ongoing high-output losses or renal/electrolyte abnormalities | Repeat basic metabolic panel and renal assessment | Monitor correction of dehydration, hypokalemia, acidosis, and kidney injury. [22][24] |

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## Editorial note

Prepared from cited clinical literature using Astra's research workflow. Verify recommendations against current guidance and patient-specific factors.