Synopsis
Separate acute infection and dehydration from chronic inflammatory, malabsorptive, secretory and functional diarrhoea, select stool and endoscopic tests by consequence, and protect patients and contacts through appropriate public-health action.
- Define true diarrhoea by increased stool liquidity and frequency, then clarify onset, duration, volume, blood, nocturnal stool, urgency, steatorrhoea, pain and fasting response.
- Acute illness is commonly infectious, medicine-related or inflammatory; symptoms lasting four weeks or more require a structured chronic-diarrhoea pathway rather than repeated empirical antibiotics.
- Ask about travel, food and water, sick contacts, antibiotics, hospital or care-home exposure, sexual exposure, immunosuppression and occupation because testing and public-health consequences differ.
- Blood, fever, severe pain, systemic toxicity and dehydration alter urgency; anti-motility treatment can be unsafe when invasive infection, C difficile or acute severe colitis is plausible.
- Review metformin, magnesium, antibiotics, laxatives, PPIs, NSAIDs, colchicine, orlistat and other medicines before labelling chronic diarrhoea idiopathic.
- Faecal calprotectin addresses intestinal inflammation in an appropriate population, quantitative FIT addresses colorectal cancer risk, and neither test substitutes for the other.
- Chronic watery diarrhoea may reflect bile acid diarrhoea or microscopic colitis even when colonoscopy looks normal; correct testing and colonic biopsies are essential.
- Fatty, difficult-to-flush stool with weight loss directs assessment towards coeliac disease, pancreatic insufficiency and small-bowel malabsorption.
- Oral rehydration replaces both salt and water more effectively than plain water during substantial gastrointestinal losses; intravenous replacement is needed when shock or oral failure develops.
Key red flags
High fever, bloody stool, severe pain, dehydration, sepsis, recent travel, outbreak exposure or immunocompromise raises invasive infection and justifies urgent sampling, isolation and specialist or public-health discussion before empirical treatment.
New unexplained diarrhoea after antibiotics, healthcare exposure or in frailty raises C difficile. Severity includes clinical and laboratory features; ileus or toxic megacolon may reduce stool output despite fulminant disease.
Blood, urgency, nocturnal diarrhoea, weight loss, fever, perianal disease, extra-intestinal inflammation and raised calprotectin support IBD, while acute severe colitis requires immediate hospital and gastroenterology assessment.
Investigation priorities
Identify bacterial, viral or parasitic infection when severity, duration, travel, outbreak or host factors make the result actionable.
Management branches
Diarrhoea began recently and may be infectious.
- Assess perfusion, urine output, electrolytes, abdominal signs and host risk, giving oral rehydration or intravenous fluid according to severity.
- Institute appropriate enteric precautions and take an exposure history covering travel, food, contacts, antibiotics, healthcare, occupation and immune status.
- Send targeted stool testing for blood, systemic illness, persistence, high-risk exposure or public-health consequence; discuss severe cases with microbiology.
- Avoid routine antibiotics and anti-motility agents until invasive, toxin-mediated and C difficile risks are considered, then safety-net hydration and worsening features.