Red flags1 exam ›
- Nocturnal diarrhoea - waking specifically to pass stool = organic, not functional
- Rectal bleeding; anaemia
- Unintentional weight loss
- Fever, night sweats
- Onset age >50 with no prior history
- Family history of IBD, coeliac, colorectal cancer
- Short history with rapid progression
- Abnormal bloods (inc CRP, dec albumin, dec Hb)
- Dehydration, hypotension, oliguria
- Severe diarrhoea + flushing, palpitations, bronchospasm, or FHx MEN1/MEN2 -> neuroendocrine tumour (carcinoid, VIPoma)
- Recent antibiotics or hospitalisation -> C. difficile
- Immunosuppression, HIV
Differential by mechanism7 exam ›
Mechanistic classification
| Mechanism | Driver | Examples |
|---|---|---|
| Secretory | dec absorption / inc secretion | Cholera toxin, ETEC, VIPoma, carcinoid, neuroblastoma, gastrinoma, bile-acid diarrhoea, microscopic colitis |
| Osmotic | Maldigestion / malabsorption, unabsorbed solute | Lactase deficiency, generalised malabsorption, laxative abuse (Mg, PEG), sorbitol |
| Inflammatory (exudative) | Mucosal destruction | IBD, invasive infection, ischaemic and radiation colitis |
| Dysmotility / dec surface area | Transit or absorptive area | Short bowel, coeliac, rotavirus enteritis, post-vagotomy, hyperthyroidism, IBS, autonomic neuropathy |
| Fatty (steatorrhoea) | Fat malabsorption | Pancreatic insufficiency, coeliac, SIBO, bile salt deficiency |
Infectious diarrhoea by pathogenic mechanism
| Mechanism | Organisms |
|---|---|
| Preformed toxin | S. aureus, B. cereus (onset 1-6 h - too fast to be an infection) |
| Secretory toxin | Cholera, ETEC, Salmonella, Shigella |
| Cytotoxin | Shigella, C. difficile, EHEC/STEC, Campylobacter |
| Direct mucosal invasion | Salmonella, Shigella, Campylobacter, Yersinia, Entamoeba |
Chronic diarrhoea - the adult short list
- IBS (diagnosis of exclusion, but positive Rome criteria)
- Coeliac disease - test everyone
- IBD
- Microscopic colitis - F >50, watery, normal mucosa, biopsy-only diagnosis
- Bile-acid diarrhoea - post-cholecystectomy, ileal resection/Crohn, idiopathic (~1/3 of "IBS-D")
- Pancreatic exocrine insufficiency
- Drugs - metformin, PPI, colchicine, magnesium, SSRIs, statins, olmesartan, laxatives, ACE inhibitors, chemotherapy
- Lactose intolerance; SIBO
- Hyperthyroidism; diabetic autonomic neuropathy
- Giardia - diarrhoea, cramps, bloating/flatulence, weight loss
- Overflow from constipation - paradoxical, and easily missed
- Neuroendocrine tumour, factitious/laxative abuse
Age shifts the list
- Infants - post-infectious secondary lactase deficiency, cows' milk/soy protein intolerance, excess fruit juice, coeliac, cystic fibrosis, congenital chloride diarrhoea, AIDS enteropathy
- Children - add IBS, lactose intolerance, giardiasis, IBD
- Adolescents - IBS, IBD, lactose intolerance, giardiasis, laxative abuse, constipation with overflow (encopresis)
- Acute in children: gastroenteritis (viral > bacterial > protozoal), food poisoning, systemic infection, antibiotic-associated; overfeeding in infants
Focused history1 exam ›
Localise: small bowel vs large bowel
| Small bowel | Large bowel (colitic) | |
|---|---|---|
| Volume | Large, watery | Small, frequent |
| Blood | Rare | Common |
| Mucus | No | Yes |
| Pain | Periumbilical, cramping | Lower abdominal, tenesmus, urgency |
| Fever | Less | More |
- Dysentery = small-volume bloody diarrhoea with mucus, tenesmus and urgency = colitic
Characterise
- Duration: acute <14 days / persistent 14-30 / chronic >30 days
- Frequency, volume, nocturnal, incontinence, urgency
- Steatorrhoea - pale, greasy, offensive, floats, hard to flush
- Relation to food; response to fasting (osmotic stops, secretory continues)
- Blood, mucus, pus
Exposure and drivers
- Travel, sick contacts, food (reheated rice - B. cereus; poultry - Campylobacter/Salmonella; unpasteurised dairy; seafood)
- Incubation: 1-6 h = preformed toxin; 8-16 h C. perfringens; >16 h invasive/viral
- Antibiotics or hospitalisation in the last 3 months -> C. difficile
- Full drug list including OTC, magnesium, laxatives, supplements
- Alcohol; sexual history; HIV risk; immunosuppression
- Surgery: cholecystectomy, ileal resection, gastrectomy, bariatric surgery
- Radiotherapy
- Family history: IBD, coeliac, CRC, MEN1/MEN2
- Diet: lactose, sorbitol/artificial sweeteners, FODMAPs, fruit juice
- Systemic: flushing, wheeze (carcinoid); heat intolerance, tremor (thyroid); rash (dermatitis herpetiformis)
Focused examination
- Volume status - postural BP, HR, mucous membranes, capillary refill, urine output
- Weight, BMI, evidence of malnutrition - temporal/interosseous wasting, oedema
- Abdomen - tenderness, mass, distension, scars, hepatomegaly, bowel sounds
- Perianal inspection + PR - fissures, tags, fistulae, faecal impaction (overflow), blood, rectal mass
- Extra-intestinal signs
- Clubbing, aphthous ulcers, erythema nodosum, pyoderma gangrenosum, episcleritis/uveitis - IBD
- Dermatitis herpetiformis - coeliac
- Flushing, tricuspid murmur - carcinoid
- Goitre, tremor, lid lag - thyrotoxicosis
- Peripheral neuropathy - B12, diabetes
- Lymphadenopathy; oedema (protein-losing enteropathy)
Investigation strategy1 exam ›
Acute
- Most acute diarrhoea needs no investigation - self-limiting
- Stool MCS + C. difficile toxin/PCR if: bloody, febrile, severe/dehydrated, immunocompromised, recent antibiotics/hospital, persisting >7 days, outbreak, food handler
- UEC, FBE, CRP, lactate if unwell
- AXR / CT if peritonism, distension or suspected toxic megacolon
Chronic - first tier
- FBE, UEC, LFT, CRP/ESR, TSH, iron studies, B12, folate, vit D, calcium
- Coeliac serology (tTG-IgA + total IgA) - in everyone, while still eating gluten
- Faecal calprotectin - separates IBD from IBS
- Stool MCS x3, C. difficile, Giardia antigen/PCR
- HIV where relevant
Second tier - directed
- Colonoscopy + biopsies (including random biopsies of normal-looking mucosa - microscopic colitis is invisible)
- Faecal elastase - pancreatic exocrine insufficiency
- SeHCAT or trial of cholestyramine - bile-acid diarrhoea
- Hydrogen breath test - lactose intolerance, SIBO
- Gastroscopy + D2 biopsies - coeliac, Whipple, giardia
- Gut hormones (VIP, gastrin, chromogranin A) + 24 h urinary 5-HIAA if secretory picture with red flags
- CT/MR enterography; faecal fat quantification
- Stool laxative screen if factitious suspected
Secretory vs osmotic - the stool test
| Secretory | Osmotic | |
|---|---|---|
| Volume | Large | Watery |
| Effect of fasting | Persists | Stops |
| Stool osmotic gap | <100 mOsm/kg | >125 mOsm/kg |
| Stool Na | High | <70 mEq/L |
| Stool pH | >6.0 | Acidic, <5.5 (carbohydrate fermentation) |
| Reducing substances | Absent | Present |
| Faecal leucocytes | Absent | Absent |
| Breath hydrogen | - | Raised with carbohydrate malabsorption |
- Osmotic gap = 290 - 2 x (stool Na + stool K)
Management
Acute infectious diarrhoea
- Rehydration is the treatment - oral rehydration solution; IV if shocked, vomiting, or unable to keep up
- Antibiotics are not routine. Consider if: severe/systemic illness, immunocompromised, >70, prosthetic material, sepsis, or a specific organism (shigellosis, amoebiasis, giardiasis, cholera, typhoid)
- Avoid antibiotics in suspected STEC/EHEC - inc risk of haemolytic uraemic syndrome
- Loperamide for symptom control - contraindicated if bloody diarrhoea, fever, or suspected C. difficile/toxic megacolon
- C. difficile - stop the precipitating antibiotic; oral vancomycin 125 mg QID 10 days first line; fidaxomicin for recurrence; faecal microbiota transplant for multiply recurrent disease. Metronidazole is no longer first line
- Notify and exclude from work (food handlers, childcare, healthcare) per public health requirements
Chronic diarrhoea - treat the cause
- Coeliac -> gluten-free diet
- IBD -> induction + maintenance immunotherapy
- Microscopic colitis -> stop the culprit drug (PPI, NSAID, SSRI); budesonide 9 mg first line
- Bile-acid diarrhoea -> cholestyramine
- Pancreatic insufficiency -> pancreatic enzyme replacement with meals + fat-soluble vitamins
- SIBO -> rifaximin; treat the underlying stasis
- IBS-D -> dietary (low FODMAP with dietitian), loperamide, amitriptyline
- Lactose intolerance -> lactose restriction
- Neuroendocrine tumour -> octreotide, resection
Everyone
- Correct dehydration and electrolytes (K+, Mg2+, bicarbonate)
- Replace deficiencies: iron, B12, folate, fat-soluble vitamins, zinc, calcium
- Review every drug on the list
Traps
- "Diarrhoea" may mean incontinence, or frequency without volume - ask what actually happens
- Overflow diarrhoea from faecal impaction - the treatment is disimpaction, not loperamide
- Normal-looking mucosa does not exclude colitis - microscopic colitis needs random biopsies
- A normal CRP does not exclude active small bowel Crohn
- Coeliac serology is falsely negative if already gluten-free or IgA-deficient - check total IgA
- Loperamide in C. difficile or acute severe colitis -> toxic megacolon
- Antibiotics for STEC -> HUS
- 1-6 h onset after food = preformed toxin, not an infection - antibiotics are useless
- Anti-motility agents mask ongoing blood loss in colitis
- Post-cholecystectomy or ileal resection diarrhoea is bile-acid diarrhoea until proven otherwise - a cholestyramine trial is cheaper than SeHCAT
- Metformin, PPIs, magnesium and olmesartan are common, reversible and routinely missed
- Thyrotoxicosis and diabetic autonomic neuropathy present as "IBS"
- Laxative abuse: think in young women, unexplained hypokalaemia, or melanosis coli at colonoscopy
Talk track
1. Frame it
- "Acute or chronic, and small bowel or large bowel."
2. Localise
- "The small-volume bloody stool with tenesmus and urgency tells me this is colitic, so I am thinking large bowel: infection, inflammatory bowel disease, ischaemia."
3. Red flags out loud
- "The features that concern me are nocturnal diarrhoea, weight loss, and anaemia - these point away from a functional cause."
4. Mechanism
- "The stool osmotic gap and the response to fasting will separate secretory from osmotic."
5. Investigate in tiers
- "First: coeliac serology, faecal calprotectin, stool studies, thyroid function. Then colonoscopy with biopsies including from normal mucosa."
6. Close on cause-directed treatment + the deficiencies
- "Rehydration and electrolytes first, then treat the cause, and replace iron, B12 and fat-soluble vitamins.
8 of 8 sections written · drafted 2026-09-03