Diarrhea: acute or chronic, and its four mechanisms
- Topic
Loose or frequent stools: how long they have lasted and what the stools look like point to infection, inflammation, malabsorption or a functional cause.
See it in 3D: Jejunum · Ileum · Sigmoid colon
What it is
Diarrhea is the passage of loose or watery stools, usually three or more times a day, or more often than is normal for the person. Duration splits the approach: acute diarrhea (up to about two weeks) is mostly infectious and self-limited, while chronic diarrhea (more than four weeks) needs a search for a cause. The mechanism (osmotic, secretory, inflammatory or malabsorptive, plus fast transit) is read from the stool and the history, and dehydration is the main danger at any age.
Key facts
- Ask: onset and duration, number and volume of stools, consistency, blood or mucus, nocturnal stools, urgency and tenesmus.
- Large-volume watery stools suggest small bowel; frequent small stools with urgency, blood and tenesmus suggest colon and rectum.
- Exposures: suspect food, people ill at home, untreated water, travel, daycare, recent antibiotics, hospital stay.
- Associated symptoms: fever, vomiting, abdominal pain and where it is, weight loss, joint pain, skin or eye inflammation.
- Chronic diarrhea background: lactose intake, sugar alcohols, laxatives, medicines, alcohol, previous surgery, thyroid symptoms, celiac disease in the family.
- Stool osmotic gap (290 minus twice the sum of stool sodium and potassium): small gap (under about 50) suggests secretory; large gap suggests osmotic diarrhea.
Step by step
- Assess hydration first: thirst, urine output, mucous membranes, skin turgor, heart rate, blood pressure, mental state; in children, tears and fontanelle.
- Establish duration: acute, persistent or chronic.
- Ask what the stool looks like: watery, bloody, mucous, greasy or pale, and whether it continues at night or when fasting.
- Ask about exposures: food, water, contacts, travel, animals, antibiotics, hospitals, sexual history.
- Look for systemic signs: fever, weight loss, joint pain, rash, mouth ulcers, perianal disease.
- Examine the abdomen for tenderness, distension and masses; examine the perianal area and rectum when indicated.
- Decide: mild acute watery diarrhea without red flags needs fluids and observation; red flags or chronic diarrhea need tests.
Diarrhea by mechanism
| Mechanism | How it behaves | Example |
|---|---|---|
| Osmotic | Stops with fasting; large osmotic gap; bloating after the trigger | Lactose intolerance, laxatives, sugar alcohols |
| Secretory | Large watery volumes, continues fasting and at night; small gap | Cholera and other toxin-producing infections |
| Inflammatory, acute infection | Blood, mucus, fever, tenesmus | Bacterial dysentery |
| Inflammatory, chronic | Bloody diarrhea for weeks, urgency, weight loss | Ulcerative colitis, Crohn disease |
| Antibiotic-associated | Watery diarrhea after antibiotics or hospital stay | Clostridioides difficile infection |
| Malabsorptive | Steatorrhea, weight loss, deficiencies | Celiac disease |
| Parasitic malabsorption | Greasy foul stools, bloating, after untreated water | Giardiasis |
| Altered motility | Pain relieved by defecation, no nocturnal stools, no weight loss | Irritable bowel syndrome |
Clinical notes
- Severe dehydration (lethargy, sunken eyes, very slow skin pinch, inability to drink, little or no urine) is an emergency, above all in infants and older adults.
- Bloody diarrhea with fever and abdominal pain suggests invasive bacterial infection; in children, watch for kidney failure and low platelets (hemolytic uremic syndrome).
- Chronic diarrhea with weight loss, nocturnal stools, blood or anemia is not irritable bowel syndrome until organic causes are excluded.
- Diarrhea with abdominal distension, fever and tachycardia after antibiotics may be severe colitis.
- In people living with HIV, chronic diarrhea needs a search for opportunistic parasites and infections.
In the lab
- Acute watery diarrhea without red flags usually needs no tests.
- With red flags: complete blood count, electrolytes, urea and creatinine; stool tests for pathogens, ova and parasites, and Clostridioides difficile when relevant.
- Chronic diarrhea: complete blood count, celiac serology, thyroid tests, fecal calprotectin, stool for Giardia, stool electrolytes and fat when the mechanism is unclear.
- Colonoscopy with biopsies when inflammatory disease or microscopic colitis is suspected.
Test yourself
Think of the answer, then tap the question to see it.
From what duration is diarrhea called chronic?
More than four weeks; up to about 14 days is acute and 14 to 30 days is persistent.
Which simple question separates osmotic from secretory diarrhea?
Whether it stops with fasting: osmotic diarrhea stops, secretory diarrhea continues, including at night.
What do blood, mucus, fever and tenesmus suggest?
Inflammatory diarrhea of the colon: bacterial dysentery or inflammatory bowel disease.
Which features of a stool suggest fat malabsorption?
Pale, bulky, greasy, foul-smelling stools that float and are hard to flush, often with weight loss.
Connections
See it in 3D3
The structures of Durgical's 3D anatomy where it is found.
Educational content on the clinical exam; it is learned and practised under supervision. It does not replace assessment by a health professional and is not for self-diagnosis.
Sources: Diarrhea (MedlinePlus Medical Encyclopedia, U.S. National Library of Medicine, 2025); Diarrhea (MSD Manual Professional Edition, 2025).
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Representative model, for educational purposes only.