Approach to diarrhoea

• Diarrhoea means loose or watery stool passed 3 or more times a day, or more often than is normal for that person; it is a symptom first and a measured weight later.
• Acute diarrhoea lasts under 2 to 3 weeks, persistent 2 to 4 weeks, and chronic 4 weeks or more; blood in the stool is dysentery.
• Mechanisms sort into osmotic, secretory, inflammatory, malabsorptive, motility and exudative; the stool osmotic gap estimates which is dominating.
• Most (roughly 80%) of acute diarrhoea is infective; the priorities are dehydration, red flags, and not harming the patient with antidiarrheals or unnecessary antibiotics.
• Chronic diarrhoea is sorted by category (drugs, food, systemic disease, infection, organic disease, bile acids, functional or irritable bowel syndrome), then investigated stepwise.

By the end of this page you should be able to:

1. Define diarrhoea, dysentery, and acute, persistent and chronic diarrhoea, and explain the main mechanisms.
2. Use the stool osmotic gap to separate secretory from osmotic diarrhoea.
3. Assess dehydration and prescribe oral rehydration solution and zinc correctly.
4. Recognise when stool tests are needed, when antibiotics are not indicated, and when antimotility drugs are dangerous.
5. Work through chronic diarrhoea by category and plan the stepwise investigations.
6. Recognise common paediatric, traveller's, and low-resource presentations and their management.

Loose stool on its own is a poor measure; normal stool frequency runs from three times a week to three times a day. By weight, adults and children pass under 200 g of stool a day, and more than that, taken over 24 to 72 hours, is an objective definition of diarrhoea. Because 20% of people with loose stools still fall under 200 g, the symptom is defined as a fall in stool consistency or an increase in frequency above what is usual.

Diarrhoea as a disease is classified by duration:

• Acute diarrhoea: under 2 to 3 weeks, rarely 6 to 8 weeks.
• Persistent diarrhoea: 2 to 4 weeks (adult and paediatric definitions differ; WHO uses 14 days or more in children).
• Chronic diarrhoea: 4 weeks or more (most textbooks use at least 4 weeks).

Dysentery is diarrhoea with blood in the stool, usually with fever and urgency; it points to colonic inflammation rather than simple secretory loss.

Mechanistically, diarrhoea results from too little absorption, too much secretion, exudation across a damaged mucosa, or altered motility. Many illnesses mix more than one mechanism. For clinical use it is practical to group them as: osmotic (poorly absorbed solute drags water into the lumen), secretory (active ion secretion), inflammatory (damaged epithelium with or without exudation and fever), malabsorptive (fat and nutrient malabsorption, often steatorrhoea), motility-related (rapid transit), and exudative (mucus, pus or blood in stool).

The osmolality of stool as passed is close to that of plasma (280 to 300 mOsm per kg water). The stool osmotic gap is the difference between that osmolality (or 290 mOsm) and twice the sum of stool sodium and potassium:

gap = stool osmolality − 2 × (stool Na + stool K)

It is a bedside estimate of how much unmeasured solute is present in the stool.

Stools with sodium between 60 and 90 mmol/L and an osmotic gap of 50 to 100 can be either secretory or malabsorptive, so treat them as indeterminate. A stool pH under 5.3 usually means pure carbohydrate malabsorption; in generalised malabsorption the pH is usually over 5.6.

Children are the highest-risk group: diarrhoea is the third leading cause of death in children aged 1 to 59 months and kills around 443 832 children under 5 years each year, with nearly 1.7 billion cases occurring in children world-wide. Malnourished and immunocompromised children are the most vulnerable. ^[2]

The 2016 ACG guideline on acute diarrheal infections in adults supports the same broad message: investigate bloody or invasive diarrhoea, water- and food-borne outbreaks and infection in travellers, and treat dehydration first. ^[7]

Ask about onset and duration, because the cut-offs (under 2 to 3 weeks acute, 2 to 4 weeks persistent, 4 weeks or more chronic) change the differential sharply. Ask about stool character in the patient's own words: watery, bloody (dysentery), fatty or hard to flush (steatorrhoea), or mucoid. Ask about diurnal pattern: pure nocturnal diarrhoea leans toward secretory disease, whereas post-prandial urgency is more typical of colonic or osmotic causes.

Ask about pain and its relation to defaecation, tenesmus and urgency, weight loss, fever, incontinence, and what the patient was eating. Recent travel, camping or fresh-water exposure, daycare contact, antibiotic exposure (especially in the preceding weeks to months), hospitalisation, and new medications are high-yield (over 700 drugs can cause diarrhoea, including proton pump inhibitors, metformin, laxatives, angiotensin-converting enzyme inhibitors, selective serotonin reuptake inhibitors, non-steroidal anti-inflammatory drugs and magnesium-containing antacids or supplements). Ask about surgery (gastrectomy, cholecystectomy, small-bowel or ileal resection), radiotherapy, endocrine disease (thyroid, diabetes, Addison disease), systemic illness (HIV, immunosuppression), and family history of inflammatory bowel disease or colorectal cancer. In women, ask about menstrual history and weight changes, which can point toward eating disorders with laxative use.

Ask specifically about laxative and alternative remedies, including herbal and diet products, because hidden laxative abuse causes a large share of chronic secretory diarrhoea; a laxative screen on urine and stool may be needed.

Examine the general state: fever, tachycardia, postural hypotension, weight loss, malnutrition, signs of vitamin or mineral deficiency (anaemia, glossitis, neuropathy, tetany). Examine the abdomen for distension, masses, tenderness, Crohn's perianal disease, surgical scars, ascites and hepatomegaly, and do a rectal examination (rectal mass, blood, mucus, sphincter tone, faecal loading).

Physical pattern matters. Dehydration is graded by the WHO system:

In straightforward acute watery diarrhoea in a healthy adult, no tests are needed; the illness is usually self-limited. Investigate when there are red flags, severe disease, hospital or antibiotic exposure, travel, or immunocompromise.

Lab tests to consider (standard approach, matching the Japanese chronic diarrhea guideline):

• Full blood count, electrolytes, urea, creatinine, albumin, CRP for inflammation, fluid status and nutritional state.
• Thyroid function (free T3, free T4, TSH) to detect hyperthyroidism.
• Stool tests: culture for Salmonella, Shigella and Campylobacter in febrile or invasive illness; Clostridioides difficile toxin if recent antibiotics, hospitalisation or peripartum; ova and parasites (Giardia, Cryptosporidium, Entamoeba, Cyclospora, Strongyloides) in travel, persistent diarrhoea or immunocompromise; norovirus and rotavirus testing in outbreaks; stool antigen for Giardia.
• Fecal occult blood to screen for neoplasia in older people and inflammatory bowel disease in the young.
• Fecal calprotectin (and lactoferrin) are raised in inflammatory bowel disease and help separate it from irritable bowel syndrome.
• Quantitative stool fat (normal under 7 g per 24 h on a 100 g fat diet; borderline 8 to 14 g in non-malabsorptive secretory or osmotic diarrhoea) and fecal elastase (low values indicate pancreatic insufficiency) for malabsorption.
• Celiac serology (tissue transglutaminase IgA, with total IgA), short synacthen test if adrenal insufficiency is suspected, and vitamins (B12, folate, iron, vitamin D, calcium) as directed by the clinical picture.
• Hydrogen breath tests for lactose malabsorption and small-intestinal bacterial overgrowth.
• Abdominal imaging (CT or MRI) for inflammatory or neoplastic disease, extraintestinal pathology and suspected bile-acid diarrhea.
• Colonoscopy with biopsies is recommended in chronic diarrhoea, including random biopsies even when mucosa looks normal, because endoscopy and biopsies are abnormal in 17 to 28% of patients meeting Rome IV criteria; it is especially justified after empiric treatment has failed, in patients over 50, and when warning signs are present. ^[5]

Routine stool culture is of little value after 3 to 5 days of hospitalisation, except in neutropenic patients, people living with HIV, or those with clear enteric infection, and even with modern testing 20 to 40% of acute infectious diarrhoeas remain undiagnosed. ^[1]

The immediate goals are rehydration, correction of electrolytes, and safe, cause-directed treatment.

Rehydration. Oral rehydration with reduced-osmolarity oral rehydration solution is first line for dehydration. The WHO/UNICEF-reduced composition used world-wide is: glucose 75 mmol/L, sodium 75 mmol/L, chloride 65 mmol/L, potassium 20 mmol/L, citrate 10 mmol/L, total osmolarity 245 mOsm/kg water. ^[3] One packet is usually reconstituted in 1 L of safe water. For children and infants, start early to prevent dehydration. Intravenous fluids (e.g., Ringer's lactate or normal saline) are reserved for severe dehydration or shock. ^[2]

Zinc in children. WHO/UNICEF recommend 10 to 20 mg of zinc per day (often 20 mg in children over 6 months and 10 mg under 6 months) for 10 to 14 days. Zinc shortens the illness by roughly 25% and reduces stool volume by about 30% where deficiency or malnutrition is present, with the greatest benefit in children over 6 months; it has little or no effect under 6 months and may cause vomiting. ^[2][4]

Feeding. Continue breast-feeding; offer a normal diet as tolerated. Nutrient-rich food matters because diarrhoea and malnutrition worsen each other.

Antimotility drugs. Loperamide 2 to 4 mg (up to 2 to 4 times daily; total adult doses up to 24 mg a day reported as safe) is useful for mild to moderate diarrhoea in adults, particularly travellers. Avoid opiates and anticholinergics when illness is severe or bloody, or when there is fever or dysentery, C. difficile infection, or severe inflammatory bowel disease, because stasis can worsen invasion and precipitate megacolon. Children under 6 years rarely receive these drugs in practice. ^[1][6]

Antibiotics. Not routinely indicated for acute watery diarrhoea. They are appropriate for bloody or febrile (invasive) diarrhoea (dysentery), C. difficile infection, and severe traveller's diarrhoea, and they shorten illness by roughly 1 to 2 days in susceptible bacterial infection. Empirical choices depend on region and resistance: azithromycin 500 mg once daily for 3 days or 1000 mg as a single or divided dose is preferred for dysentery and in South or South-East Asia; fluoroquinolones such as ciprofloxacin are now limited by resistance in Campylobacter and Shigella; rifaximin 200 mg three times daily or rifamycin SV for non-invasive cases only. ^[6]

Racecadotril, an enkephalinase inhibitor, reduces intestinal secretion without causing stasis and is an alternative where available.

Chronic diarrhoea is usefully classified into eight categories: drug-induced; food-induced; systemic disease-associated; infection-associated; organic (inflammatory or neoplastic); bile acid diarrhoea; functional; and diarrhoea-predominant irritable bowel syndrome. The Japanese gastroenterological guideline notes that AGA (2019, succeeding the 2018 British guideline) and Rome IV criteria set the chronic cut-off at 4 weeks, which matches clinical practice better than the 6-month Rome window. ^[5]

Examples by mechanism and category:

Stepwise workup (matching the Japanese chronic diarrhea guideline and the standard Western approach): ^[5][1]

1. Detailed history (onset, drugs, surgery, diet, travel, family, psychosocial).
2. Physical examination and basic labs: full blood count, electrolytes, albumin, urea, CRP, coagulation; thyroid; celiac serology; HIV where indicated.
3. Stool studies: calprotectin (raised with inflammatory bowel disease), culture, parasites, elastase, fat, and a laxative screen if factitious diarrhea is suspected.
4. Imaging and endoscopy: abdominal CT when indicated; colonoscopy with biopsies for inflammation, microscopic colitis or neoplasia; upper endoscopy and small-bowel biopsy if celiac disease or malabsorption is suspected.
5. Functional and targeted tests: hydrogen breath tests for carbohydrate intolerance and bacterial overgrowth; SeHCAT, C4 or FGF-19 for bile acid diarrhoea; 24-hour urine 5-HIAA for carcinoid; vasoactive intestinal peptide and gastrin levels for neuroendocrine tumour.

Warning symptoms that make organic disease more likely are: unexpected weight loss, nocturnal diarrhoea, recent antibiotics, bloody stools, frequent or profuse diarrhoea, undernutrition, and a family history of inflammatory bowel disease or colorectal cancer. Their usefulness on its own is uncertain, so a normal screen does not replace judgment. ^[5]

The Japanese guideline recommends colonoscopy for chronic diarrhoea to exclude organic disease, including random biopsies throughout the colon for microscopic colitis, eosinophilic gastroenteritis, and amyloidosis, when systemic clues are absent on endoscopy. It is especially appropriate for empiric-treatment-resistant cases, patients over 50, and those with warning signs. ^[5]

Children are the group at highest risk worldwide. The common viral causes in young children are rotavirus, norovirus, and enteric adenovirus (with astrovirus), usually as winter or sporadic acute watery disease that is self-limited. Bacterial causes include Escherichia coli (enterotoxigenic, enteropathogenic, enterohaemorrhagic), Salmonella, Shigella and Campylobacter; parasitic causes include Giardia, Cryptosporidium, Cyclospora and Entamoeba. ^[2][6]

Persistent diarrhoea in a child (14 days or more) requires active management to prevent malnutrition. Careful hydration, continued feeding, and correction of zinc deficiency are central; antibiotics are used only when bacterial or parasitic infection is established or strongly suspected.

Traveller's diarrhoea attack rates range from 30% to 70% during a 2-week period, depending on destination, season, food-handling and sanitation. Bacteria predominate (about 75% to 90%), viruses cause 10% to 25% (often with more vomiting), and protozoa about 10%, especially in longer trips and more persistent illness. Bacterial diarrhoea, untreated, usually lasts 3 to 7 days; viral 2 to 3 days; protozoal illness weeks to months. ^[6]

Management follows severity (CDC Yellow Book, 2026):

• Mild: tolerable, does not interfere with activities; avoid antibiotics; consider bismuth subsalicylate or loperamide.
• Moderate: distressing or interferes with activities; antibiotics can be used, with loperamide as monotherapy or adjunct.
• Severe or dysenteric: incapacitating or bloody; antibiotics are advised (single-dose possible), and loperamide only as adjunct, not as monotherapy, never in bloody diarrhoea or with fever. ^[6]

Empirical antibiotics: azithromycin 500 mg once daily for 3 days or 1000 mg single or divided dose (preferred in South and South-East Asia and for dysentery), ciprofloxacin 500 mg twice daily or 750 mg single dose, levofloxacin 500 mg once daily for 1 to 3 days, rifaximin 200 mg three times daily for 3 days, or rifamycin SV 388 mg twice daily for 3 days (non-invasive disease only). They reduce duration by about 1 to 2 days. Loperamide may be combined: 4 mg initially, then 2 mg after each loose stool, not exceeding 16 mg in 24 hours. ^[6]

Prevention rests on careful food and drink choices and hand hygiene; bismuth subsalicylate prophylaxis reduces incidence by about 50% but is not recommended under 3 years or in pregnancy, or with salicylate use. Prophylactic antibiotics are discouraged for most travellers because of resistance, side effects and Clostridioides difficile. ^[6]

In low-resource practice, extra attention goes to: identifying dehydration by the skin pinch and ability to drink; using ORS prepared from one packet in 1 L of safe water; avoiding antidiarrheals in suspected cholera; treating dysentery with appropriate antibiotics where bacterial causes are likely; and screening persistently malnourished children for other causes (malnutrition itself worsens and is worsened by diarrhoea).

• Diarrhoea is defined by consistency or frequency; acute under 2 to 3 weeks, persistent 2 to 4 weeks, chronic 4 weeks or more.
• Dysentery means blood with diarrhoea and implies colonic inflammation.
• The four classic mechanisms are osmotic, secretory, exudative and motility-related; add malabsorptive and inflammatory injury as useful clinical categories.
• The stool osmotic gap (290 minus 2 times sodium plus potassium) is under 50 in secretory and over 125 in osmotic diarrhoea; values in between (50 to 100) are indeterminate.
• Around 80% of acute diarrhoea is infective; dehydration and red flags come first in management.
• Reduced-osmolarity ORS (75/75/65/20/10 mmol/L, 245 mOsm/kg water) is standard, with IV fluids for severe dehydration.
• Zinc, 10 to 20 mg per day for 10 to 14 days, shortens diarrhoea in children over 6 months where deficiency exists; it helps least under 6 months.
• Loperamide is useful for mild to moderate diarrhoea in adults but is dangerous in bloody, febrile or severe inflammatory disease.
• Antibiotics are not routinely needed in acute watery diarrhoea; they are justified in dysentery, C. difficile infection, and severe traveller's diarrhoea.
• Chronic diarrhoea is sorted into drug, food, systemic, infection, organic, bile acid, functional and IBS categories, then investigated stepwise with blood tests, stool studies, calprotectin, elastase, celiac serology, and colonoscopy with biopsies.
• Children under 5, especially when malnourished, and adults over 70 carry the highest risk of death from diarrhoea.
• In traveller's diarrhoea, bacteria dominate, protozoa are more likely with persistence, and treatment is matched to severity.