Diarrhea
Diarrhea is three or more unformed stools daily, split by duration into acute (under 14 days, usually infectious), persistent (14 to 30 days, often parasitic), and chronic (over 30 days, usually noninfectious), and by mechanism into osmotic, secretory, inflammatory, malabsorptive, and motility diarrhea. Acute diarrhea is managed with rehydration and tested only when red-flag features are present, while chronic diarrhea is worked up by mechanism using stool studies, celiac serology, fecal calprotectin, and endoscopy.
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What this article covers
- Overview
Definition: diarrhea is a decrease in stool consistency or an increase in stool frequency, operationally defined as three or more unformed stools per day, or a stool weight above 200 g (200 mL) per 24 hours on a Western diet.
- Epidemiology
Acute diarrhea is overwhelmingly infectious: viruses (norovirus foremost), bacteria, and parasites cause most cases.
- Risk factors
Recent antibiotic or hospital exposure: the leading risk factor for Clostridioides difficile infection (Clostridioides difficile infection).
- Pathophysiology
Osmotic diarrhea: a poorly absorbed, osmotically active substance (unabsorbed lactose, fructose, sorbitol, magnesium antacids, saline cathartics) retains luminal water; it depends on ongoing…
- Clinical features
Stool pattern localizes the mechanism: diarrhea occurring with eating and resolving with fasting suggests osmotic diarrhea; diarrhea continuing despite fasting suggests secretory diarrhea; nocturnal…
- Differential diagnosis
Clostridioides difficile infection: watery or mucoid diarrhea after antibiotic or hospital exposure; confirmed by stool toxin/PCR (Clostridioides difficile infection).
- Diagnosis
The diagnosis of diarrhea itself is clinical (3 or more unformed stools per day, or stool weight over 200 g/24 h); the diagnostic task is determining duration category and mechanism, not confirming "diarrhea" on a test.
- Management
Rehydration is the cornerstone of treatment for every cause of diarrhea: oral rehydration solution for most patients, escalating to IV balanced crystalloid for severe volume depletion or inability to…
- Complications
Severe dehydration and hypovolemic shock: the most immediate life threat, especially in young children, older adults, and cholera-like high-volume disease.
- Prognosis
Acute infectious diarrhea is generally self-limited, resolving within days with supportive care alone in most healthy adults.
- Prevention
Safe food and water for travel: avoid tap water and ice, undercooked meat, unpasteurized dairy, and unpeeled fruit; disinfect water by boiling 3 minutes or with chlorine or iodine.
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Related articles
- Clostridioides difficile infection
Clostridioides difficile infection is toxin-mediated colitis that follows disruption of normal colonic flora, most often by antibiotics, and ranges from watery diarrhea to fulminant colitis with shock, ileus, or megacolon. Diagnosis rests on unformed stool testing (NAAT and/or toxin EIA) in a patient with three or more unformed stools in 24 hours, and treatment is chosen by IDSA/SHEA severity grade, oral vancomycin or fidaxomicin for 10 days, escalating to high-dose vancomycin plus IV metronidazole for fulminant disease.
- Giardiasis
Giardiasis is infection of the proximal small bowel with the protozoan Giardia lamblia (duodenalis), acquired from cysts in contaminated fresh water or food, causing malabsorptive, foul-smelling, greasy diarrhea with bloating and weight loss over weeks. Diagnosis rests on a stool antigen or nucleic acid test, and treatment is a short course of tinidazole, metronidazole, or nitazoxanide.
- Celiac disease
Celiac disease is an immune-mediated small-intestinal enteropathy triggered by dietary gluten in genetically predisposed individuals, presenting with malabsorptive diarrhea, iron-deficiency anemia, or dermatitis herpetiformis. Diagnosis requires IgA tissue transglutaminase serology with a total IgA level while the patient still eats gluten, confirmed by duodenal biopsy showing villous atrophy, and the only treatment is a strict lifelong gluten-free diet.
- Irritable bowel syndrome (IBS)
Irritable bowel syndrome is a functional disorder of gut-brain interaction diagnosed clinically by Rome IV criteria, recurrent abdominal pain related to defecation, stool frequency, or stool form, without alarm features. Management is subtyped by stool pattern, with fiber and dietary change first, then IBS-C secretagogues or IBS-D antidiarrheals and antibiotics, and low-dose tricyclics or antispasmodics for pain.
- Foodborne illness (food poisoning)
Foodborne illness is acute gastrointestinal disease from a contaminated food, and the incubation period after the meal is the single fastest clue to the pathogen, splitting a short 1-6 hour vomiting-predominant preformed-toxin illness from an 8-16 hour toxin-mediated diarrhea and a longer than 16 hour invasive, often febrile or bloody, illness. Management rests on oral rehydration for nearly everyone, with antibiotics reserved for specific pathogens or severe disease and withheld in suspected enterohemorrhagic E coli because they raise the risk of hemolytic uremic syndrome.