[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Clinical exam](https://durgical.com/en/clinical-exam/) › [Abdomen](https://durgical.com/en/clinical-exam/areas/abdomen/) › Diarrhea: acute or chronic, and its four mechanisms

# 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.

Updated Nov 13, 2026

**See it in 3D:** [Jejunum](https://durgical.com/en/anatomy/jejunum/) · [Ileum](https://durgical.com/en/anatomy/ileum/) · [Sigmoid colon](https://durgical.com/en/anatomy/sigmoid-colon/)

## Key points

- Acute: up to about 14 days; persistent: 14 to 30 days; chronic: more than four weeks.
- Osmotic diarrhea stops with fasting; secretory diarrhea continues with fasting and at night.
- Blood, pus, fever and tenesmus mean inflammatory diarrhea (dysentery or inflammatory bowel disease).
- Pale, bulky, greasy, foul stools that float and are hard to flush suggest fat malabsorption (steatorrhea).
- Recent antibiotics or hospital stay: think of Clostridioides difficile.
- Red flags: dehydration, blood in stool, high fever, severe pain, nocturnal diarrhea, weight loss, age extremes, immunosuppression.

## 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](https://durgical.com/en/conditions/lactose-intolerance/) | Stops with fasting; large osmotic gap; bloating after the trigger | Lactose intolerance, laxatives, sugar alcohols |
| [Secretory](https://durgical.com/en/conditions/cholera/) | Large watery volumes, continues fasting and at night; small gap | Cholera and other toxin-producing infections |
| [Inflammatory, acute infection](https://durgical.com/en/conditions/bacterial-infectious-diarrhoea-and-dysentery/) | Blood, mucus, fever, tenesmus | Bacterial dysentery |
| [Inflammatory, chronic](https://durgical.com/en/conditions/ulcerative-colitis/) | Bloody diarrhea for weeks, urgency, weight loss | Ulcerative colitis, Crohn disease |
| [Antibiotic-associated](https://durgical.com/en/conditions/clostridioides-difficile-infection/) | Watery diarrhea after antibiotics or hospital stay | Clostridioides difficile infection |
| [Malabsorptive](https://durgical.com/en/conditions/coeliac-disease/) | Steatorrhea, weight loss, deficiencies | Celiac disease |
| [Parasitic malabsorption](https://durgical.com/en/conditions/giardiasis/) | Greasy foul stools, bloating, after untreated water | Giardiasis |
| [Altered motility](https://durgical.com/en/conditions/irritable-bowel-syndrome/) | 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.

## Memory aid

Fast and the stool stops: it was what went in. Fast and it keeps coming: it is what the gut is pouring out.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/clinical-exam/diarrhea-acute-or-chronic-and-its-four-mechanisms/)

## Connections

## See it in 3D 3

The structures of Durgical's 3D anatomy where it is found.

- [Jejunum](https://durgical.com/en/anatomy/jejunum/)
- [Ileum](https://durgical.com/en/anatomy/ileum/)
- [Sigmoid colon](https://durgical.com/en/anatomy/sigmoid-colon/)

## Related 6

- [Hydration status: signs of dehydration](https://durgical.com/en/clinical-exam/hydration-status-signs-of-dehydration/)
- [Dehydration and fluid overload](https://durgical.com/en/clinical-exam/dehydration-and-fluid-overload/)
- [Abdominal auscultation: bowel sounds and bruits](https://durgical.com/en/clinical-exam/abdominal-auscultation-bowel-sounds-and-bruits/)
- [Nausea and vomiting: causes by mechanism and fluid-electrolyte effects](https://durgical.com/en/clinical-exam/nausea-and-vomiting-causes-by-mechanism-and-fluid-electrolyte-effects/)
- [Unintentional weight loss and weight gain: the approach](https://durgical.com/en/clinical-exam/unintentional-weight-loss-and-weight-gain-the-approach/)
- [Digital rectal examination](https://durgical.com/en/clinical-exam/digital-rectal-examination/)

## Related conditions 8

- [Acute (viral) gastroenteritis](https://durgical.com/en/conditions/acute-gastroenteritis/)
- [Bacterial infectious diarrhoea and dysentery](https://durgical.com/en/conditions/bacterial-infectious-diarrhoea-and-dysentery/)
- [Clostridioides difficile infection](https://durgical.com/en/conditions/clostridioides-difficile-infection/)
- [Coeliac disease](https://durgical.com/en/conditions/coeliac-disease/)
- [Irritable bowel syndrome](https://durgical.com/en/conditions/irritable-bowel-syndrome/)
- [Crohn disease](https://durgical.com/en/conditions/crohn-disease/)
- [Ulcerative colitis](https://durgical.com/en/conditions/ulcerative-colitis/)
- [Giardiasis](https://durgical.com/en/conditions/giardiasis/)

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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](https://medlineplus.gov/ency/article/003126.htm) (MedlinePlus Medical Encyclopedia, U.S. National Library of Medicine, 2025); [Diarrhea](https://www.msdmanuals.com/professional/gastrointestinal-disorders/symptoms-of-gastrointestinal-disorders/diarrhea) (MSD Manual Professional Edition, 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/clinical-exam/diarrhea-acute-or-chronic-and-its-four-mechanisms/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/clinical-exam/diarrhea-acute-or-chronic-and-its-four-mechanisms/