Last updated: September 11, 2023

Summarytoggle arrow icon

Diarrhea, defined as three or more loose stools per day or more frequent stool passage than is normal for the individual, is a common disease with a monthly prevalence of around 5% in the United States. Acute diarrhea lasts 14 days or less and is almost always caused by an infectious agent, typically a virus. Testing is seldom required, as the disease tends to be self-limited and care is primarily supportive. Persistent diarrhea, lasting 15–30 days, is also typically caused by an infectious agent, but stool testing for pathogens is generally warranted to confirm the diagnosis. Chronic diarrhea, defined as diarrhea lasting longer than 30 days, has an extensive differential diagnosis. A thorough history followed by systematic stool and blood testing is required to identify the specific etiology. The presence of any red flags for severe or rapidly progressing diarrheal disease (e.g., sepsis, extreme dehydration, blood in the stool) mandates escalation of testing and, in some cases, empiric antibiotics for bacterial gastroenteritis. Ensuring adequate oral hydration, correcting dehydration, and providing symptomatic relief are fundamental components in the care of all patients with diarrhea. Curative treatment is directed toward the underlying cause.

Definitiontoggle arrow icon

The WHO defines diarrhea as ≥ 3 loose or watery stools per day or more frequent passage than is normal for the individual. [1][2]

Etiologytoggle arrow icon

For etiologies of acute, persistent, or chronic diarrhea, see specific subsections below.

Overview of pathogens causing watery and bloody diarrhea

Type of diarrhea Pathogen Pathogen characteristics Associated disorders
Watery diarrhea
  • Straight or curved (comma-shaped)
  • Cholera toxin (enterotoxin): “rice water” diarrhea
  • Transmission via contaminated water or uncooked seafood (e.g., raw shellfish)
  • Transmission via ingestion of contaminated water (e.g., lakes, rivers, ponds, swimming pools)
  • Most commonly affects hikers or campers
Bloody diarrhea
  • Curved or spiral‑shaped with polar flagellum
  • Grows best at 37–42°C
  • No lactose fermentation
  • Flagellar motility
  • Hosts: humans, animals, and animal products (e.g., reptiles, poultry, pets, eggs)
  • Slow/absent lactose fermentation
  • Shiga toxin
  • Low infectious dose required (low ID50)
  • Humans are the only host.
  • Reservoir: contaminated pork and milk products

Infectious causes [3][4]

Noninfectious [6][7][8]

Classificationtoggle arrow icon

Diarrhea is often classified as watery, fatty, and/or inflammatory to facilitate diagnosis and management. This classification scheme is most relevant for the approach to chronic diarrhea.

Classification of diarrhea by functional pathology [9][10][11]
Pathophysiology Associated disorders
Inflammatory diarrhea
  • Damage to the intestinal mucosa may cause cytokine-induced water hypersecretion, impair absorption of osmotically active substances or fat, and/or disrupt water and electrolyte absorption.
  • Mucus, blood, and leukocytes present in the stool.
Fatty diarrhea Malabsorption
  • Alterations of the intestinal mucosa → impaired absorption of digested food
Watery diarrhea Secretory diarrhea
  • Active secretion of water into the intestinal lumen via inhibition or activation of enzymes (e.g., cAMP activity)

Osmotic diarrhea

  • Poor absorption or excessive ingestion of hydrophilic substances (e.g., salts, sugars, laxatives) causes water to be drawn into the intestinal lumen.
Functional diarrheal disorders [9]
  • Rapid intestinal passage due to increased bowel activity

Managementtoggle arrow icon

Approach to diarrhea [10]

Though most patients with diarrhea have mild symptoms and can be managed as outpatients, some patients may present with severe and even life-threatening symptoms that require hospitalization.

In older adults, rule out fecal impaction, which can manifest atypically with paradoxical diarrhea (due to decreased rectal sensation) and nonspecific symptoms (e.g., functional decline, delirium). [13]

Antibiotics should only be utilized for specific indications.

Clinical assessment of diarrhea [9][11]

Key features of the disease presentation that facilitate efficient testing and prompt diagnosis include:

  • Duration of diarrheal illness
    • Acute diarrhea: duration ≤ 14 days
    • Persistent diarrhea: duration 15–30 days
    • Chronic diarrhea: duration > 30 days
  • Stool characteristics: watery, fatty, or bloody stools
  • Associated symptoms
Characteristic clinical features in diarrhea according to functional pathology
Inflammatory diarrhea
  • Frequent and small volume stools
  • Bloody or purulent stools
  • Abdominal pain, tenesmus
  • May be associated with fever, weight loss, fatigue
Fatty diarrhea
  • Greasy or oily stools
  • Stools that are difficult to flush
  • Decreases with fasting
  • May be associated with weight loss
Watery diarrhea Secretory diarrhea Loose, watery stools
  • No change with fasting
  • Nocturnal diarrhea present
Functional diarrheal disorders
  • Decreases with fasting
  • Decreased at night
Osmotic diarrhea
  • Decreases with fasting
  • Decreased at night
  • Food triggers

Risk factors for diarrhea

Assess for the presence of risk factors for specific etiologies, including:

More than 700 medications can cause diarrhea and, therefore, the introduction of a new medication within 6–8 weeks of the onset of diarrhea should be considered as a potential cause. [14]

Chronic diarrhea is common after bariatric surgery. [18][19]

Red flags in diarrhea [20][21][22][23]

Diagnosticstoggle arrow icon

General principles [9][11][21][24]

Diagnostic testing is seldom indicated in acute diarrhea in the absence of red flag symptoms.

Testing for diarrhea

For specific indications for testing, see the respective subsections below on acute, persistent, and chronic diarrhea.

Endoscopic studies have limited diagnostic value in acute diarrhea but are commonly needed for the workup of chronic diarrhea to evaluate for the presence of inflammatory or neoplastic diseases. Their utility in persistent diarrhea is uncertain.

Stool diagnostic studies

Stool diagnostic studies in diarrhea
Suspected etiology Test Purpose
Infectious diseases Stool culture Identification of infectious agents (e.g., in bacterial gastroenteritis)
Stool microscopy Visualization of ova or parasites in the stool
Culture-independent methods (e.g., stool molecular diagnostic panels) Identification of infectious agents in bacterial, viral, and protozoal gastroenteritis [24]
Quantitative culture of small intestinal aspirate Used to diagnose SIBO (gold standard test for bacterial overgrowth)
Stool tests for C. difficile infection (e.g., toxin test) Identification of C. difficile infection
Inflammation Stool microscopy Visualization of fecal leukocytes
Fecal occult blood test (FOBT) To screen for malignancy and mucosal inflammation
FIT DNA test To screen for colonic cancer and adenomatous polyps
Fecal calprotectin and lactoferrin Markers of inflammatory bowel disease (IBD)
Malabsorption Sudan stain Qualitative test for steatorrhea
72-hour quantitative fecal fat estimation Quantitative test for steatorrhea
Fecal elastase-1 Confirmation of steatorrhea due to exocrine pancreatic insufficiency
Stool electrolytes (e.g., Na+, K+) Used to calculate stool osmotic gap
Stool pH Identification of carbohydrate malabsorption
Low stool osmotic gap High stool osmotic gap
Osmotic gap
  • < 50 mmol/L
  • ≥ 100 mmol/L
  • Osmotic diarrhea (osmotic pull of ingested substances draws water into the intestinal lumen)
Example causes

The loss of bicarbonate-rich fluid in severe diarrhea may cause nonanion gap metabolic acidosis.

Supportive caretoggle arrow icon

Dehydration and hypovolemia [20]

Therapy is guided by clinical features of dehydration and hypovolemia and laboratory findings in dehydration and hypovolemia.

Acute diarrhea is usually a self-limited viral infection and management primarily supportive.

Early rehydration is especially important in children.

Antidiarrheal agents [15][24]

Loperamide should be avoided in patients with suspected invasive diarrhea with inflammatory features, as it reduces intestinal motility, which consequently increases the risk of bacterial colonization and invasion. [29]

Acute or persistent diarrhea (≤ 30 days)toggle arrow icon

Approach to acute diarrhea (≤ 14 days) [20][21][24][27]

The management of acute diarrhea is primarily supportive, as the disease is usually a self-limited viral infection. Further management focuses on determining whether diagnostic testing and directed therapy are needed (e.g., antibiotic therapy for bacterial gastroenteritis). [21]

If symptoms of an acute abdomen are present, consider an urgent CT abdomen to identify ischemic, hemorrhagic, obstructive, and/or inflammatory diseases. [27]

When indicated, stool studies in patients with acute diarrhea should include tests that can detect the presence of Salmonella, Shigella, Campylobacter, Yersinia, C. difficile infection, and Shiga toxin-producing E. coli. [30]

Noninflammatory diarrhea (watery diarrhea) [20][24][30]

  • Mild symptoms (e.g., daily activities of life are not affected)
    • < 7 days: Diagnostic testing is not recommended.
    • ≥ 7 days: Consider stool studies for pathogen identification if the results may alter management, e.g., initiation of targeted therapy.
  • Moderate to severe symptoms (e.g., daily activities of life are affected)
  • Severe symptoms, fever, or patients at risk for severe disease: Obtain stool studies for pathogen identification and targeted therapy.

Inflammatory diarrhea (dysentery) [27]

Approach to persistent diarrhea (15–30 days) [24][27][31]

In patients with prominent abdominal pain and/or bloody diarrhea, evaluate for ischemic colitis, ulcerative colitis, and Crohn disease if stool testing does not indicate an infectious etiology.

Persistent diarrhea has a broader differential diagnosis than acute diarrhea. Parasitic infection (e.g., giardiasis), recurrent bacterial infection (e.g., C. difficile infection), and noninfectious causes (e.g., IBS-D) must be considered. [31]

Etiology of acute and persistent diarrhea [9][10][14][21]

Chronic diarrhea (> 30 days)toggle arrow icon

Approach to chronic diarrhea

The differential diagnosis of chronic diarrhea is extensive. A systematic approach with efficient use of testing is needed to arrive at a diagnosis quickly. [9][11][32]

Chronic diarrhea can be classified as inflammatory, fatty, or watery. Grouping patients into broad categories based on clinical features and basic laboratory findings narrows the differential diagnosis and facilitates efficient advanced testing. [17]

IBS-D is an intermediate diagnosis; it does not eliminate the possibility of another disease. Evaluation should continue if symptoms persist despite treatment for IBS-D. [11]

Etiology of chronic diarrhea [11]

Chronic diarrhea is commonly classified as watery, fatty, or inflammatory.

Etiology of chronic diarrhea [11]
Fatty Malabsorption
Watery Secretory

Initial diagnostics for chronic diarrhea [9][11][23][32]

Order initial studies. See also “Testing for diarrhea” in “Diagnostics.”

Always test for Giardia infection in chronic diarrhea. [23]

Subsequent evaluation [9][10]

Perform guided diagnostics based on characteristic clinical features in diarrhea and the results of initial diagnostic testing.

Diagnostic workup of patients with chronic diarrhea
Classification Initial results Further diagnostics
Inflammatory diarrhea
Fatty diarrhea [9][11][15]
Watery diarrhea [10] Secretory diarrhea or functional diarrheal disorders
Osmotic diarrhea

In watery diarrhea, the stool osmotic gap differentiates osmotic from secretory diarrhea. In osmotic watery diarrhea, a stool pH < 6 suggests carbohydrate malabsorption.

Endoscopic studies with mucosal biopsies are required to diagnose inflammatory diarrhea in patients with chronic diarrhea.

Therapeutic trials [9][10][17][23]

Differential diagnosestoggle arrow icon


Subtypes and variantstoggle arrow icon

Traveler's diarrhea [35][36]

Antibiotic treatment is not recommended in patients with mild traveler's diarrhea.

Pharmacological prophylaxis [24][35]

Consider for travelers to high-risk areas (under specialist guidance); options include:

Factitious diarrhea [10]

Laxative abuse

Bile acid diarrhea

Referencestoggle arrow icon

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