Red flags1 exam ›
Refer for urgent colonoscopy
- Rectal bleeding, especially dark blood mixed with stool
- Unexplained iron deficiency anaemia - in any man, or any post-menopausal woman, this is colorectal cancer until proven otherwise
- Unintentional weight loss
- New change in bowel habit persisting >6 weeks in anyone over 50 (and increasingly under 50 - early-onset CRC incidence is rising)
- Palpable abdominal or rectal mass
- Family history of colorectal cancer or a known polyposis/Lynch syndrome
- Tenesmus, narrowed stool calibre
Acute red flags
- Bloody diarrhoea + fever + systemic upset -> severe colitis (IBD flare, C. difficile, ischaemic colitis, infective)
- Truelove and Witts severe UC: >=6 bloody stools/day plus any of HR >90, temp >37.8C, Hb <105 g/L, ESR >30 -> admit
- Abdominal distension + vomiting + absolute constipation -> obstruction or pseudo-obstruction
- Toxic megacolon - transverse colon >6 cm with systemic toxicity
- Nocturnal diarrhoea or incontinence -> organic, never functional
In a child
- Failure to thrive, weight loss, abdominal pain, vomiting, and a persistent anal fissure or fistula are red flags suggesting an organic cause of constipation
- Perianal fistula in a child - think Crohn disease
- Also: delayed passage of meconium >48 h (Hirschsprung), abnormal neurology or sacral dimple, bilious vomiting, bloody diarrhoea
Differential by mechanism1 exam ›
Diarrhoea by mechanism
| Mechanism | Feature | Causes |
|---|---|---|
| Osmotic | Stops with fasting, high stool osmotic gap (>100) | Lactose intolerance, FODMAPs, magnesium, lactulose, sorbitol |
| Secretory | Persists with fasting, nocturnal, low osmotic gap | VIPoma, carcinoid, gastrinoma, bile acid diarrhoea, microscopic colitis, cholera |
| Inflammatory | Blood, mucus, fever, raised calprotectin | IBD, infective colitis, ischaemic colitis, radiation |
| Malabsorptive/steatorrhoea | Pale, greasy, offensive, floats; weight loss, fat-soluble vitamin deficiency | Coeliac, pancreatic insufficiency, SIBO, Whipple, short gut |
| Dysmotility | Alternating, urgency without organic markers | IBS, hyperthyroidism, diabetic autonomic neuropathy |
| Drug | Temporal relationship | Metformin, PPI, magnesium, colchicine, SSRIs, antibiotics, checkpoint inhibitors, laxatives |
Constipation by mechanism
- Normal transit (functional) - the majority; IBS-C
- Slow transit - idiopathic, drugs, hypothyroidism, hypercalcaemia, hypokalaemia, Parkinson disease, autonomic neuropathy
- Defecatory disorder / dyssynergia - pelvic floor dysfunction, rectocele, rectal prolapse
- A very common and very missed cause. Laxatives will not fix it - biofeedback will
- Mechanical obstruction - colorectal cancer, stricture, volvulus, adhesions, extrinsic mass
- Drug-induced - opioids, anticholinergics, iron, calcium channel blockers, calcium and aluminium antacids, ondansetron
- Systemic - hypothyroidism, hypercalcaemia, hypokalaemia, diabetes, pregnancy, scleroderma, amyloid
- Neurological - spinal cord lesion, cauda equina, MS, Hirschsprung (paediatric)
Alternating / mixed
- IBS - the commonest, but a diagnosis made on positive criteria after excluding organic disease
- Colorectal cancer - the alternative that must not be missed
- Intestinal pseudo-obstruction - abdominal distension, vomiting and constipation in the absence of a mechanical obstructing lesion
- Acute (Ogilvie syndrome) - elderly, post-operative, post-trauma, electrolyte disturbance, opioids
- Chronic - scleroderma, amyloid, paraneoplastic, mitochondrial (MNGIE), diabetic autonomic neuropathy
- Faecal impaction with overflow (presents as diarrhoea - do a rectal examination)
- Coeliac disease, microscopic colitis, bile acid diarrhoea, diverticular disease
Focused history
Define what the patient means
- "Diarrhoea" and "constipation" mean different things to different people - ask about frequency, consistency (Bristol stool chart), urgency, effort, completeness of evacuation, and incontinence
- What has changed, and when exactly?
Discriminating features
- Nocturnal symptoms, waking from sleep -> organic
- Blood - fresh on the paper (anorectal) vs mixed through stool (colonic) vs melaena (upper)
- Mucus, tenesmus, urgency -> rectal pathology
- Relationship to fasting (osmotic vs secretory), to specific foods, to stress
- Relief of pain with defecation, and change in stool form or frequency at symptom onset -> IBS (Rome IV)
- Steatorrhoea - floating, greasy, hard to flush
Cause-seeking
- Full drug history including over-the-counter, laxatives, magnesium, PPI, metformin, recent antibiotics
- Diet - FODMAPs, artificial sweeteners (sorbitol in sugar-free products), caffeine, alcohol, fibre intake
- Travel, food-borne exposure, sick contacts, recent hospitalisation (C. difficile)
- Weight, appetite, systemic symptoms
- Previous abdominal or pelvic surgery, radiotherapy, cholecystectomy (bile acid diarrhoea)
- Family history of CRC, IBD, coeliac disease
- Obstetric history and perineal trauma (incontinence)
- Psychosocial: anxiety, depression, history of abuse - strongly associated with functional GI disorders
Focused examination
- Weight, BMI, nutritional state, hydration
- Signs of anaemia, clubbing (IBD, coeliac), mouth ulcers, glossitis, angular stomatitis
- Thyroid; skin - dermatitis herpetiformis, erythema nodosum, pyoderma gangrenosum, flushing (carcinoid)
- Abdomen: distension, tenderness, mass, hepatomegaly, succussion splash, bowel sounds (high-pitched in obstruction, absent in ileus)
- Digital rectal examination is mandatory - the most commonly omitted part of this assessment
- Faecal impaction, mass, blood, anal tone, perianal disease (fistulae, tags, abscess - Crohn), fissure
- Assess for dyssynergia: does the anal sphincter and puborectalis paradoxically contract on bearing down?
- Peripheral neuropathy, postural BP (autonomic)
Investigation strategy
Baseline in almost everyone
- FBE, ferritin/iron studies, CRP, ESR, UEC, LFT, calcium, TSH, glucose/HbA1c
- Coeliac serology (tTG-IgA with total IgA) - on a gluten-containing diet
- Faecal calprotectin - separates IBD from IBS
- <50 microg/g makes IBD unlikely; >150-250 warrants colonoscopy
- Raised by NSAIDs, infection, polyps, age and PPIs - it is a triage tool, not a diagnosis
For suspected constipation-predominant IBS
- Basic investigations to exclude organic disease before diagnosis: FBE, thyroid function tests, serum calcium, and colonoscopy
Diarrhoea
- Stool MCS, ova/cysts/parasites, C. difficile toxin PCR
- Faecal elastase (pancreatic insufficiency)
- Consider: SeHCAT or 7-alpha-hydroxy-4-cholesten-3-one for bile acid diarrhoea, hydrogen/methane breath test for SIBO or lactose intolerance
- Secretory/refractory: gut hormone panel (VIP, gastrin), urinary 5-HIAA, chromogranin A
Endoscopy
- Colonoscopy for any red flag, age >45-50 with a persistent change, raised calprotectin, or iron deficiency
- Take random biopsies from macroscopically normal mucosa in chronic watery diarrhoea - microscopic colitis is invisible endoscopically
- Gastroscopy with D2 biopsies if coeliac serology positive or malabsorption suspected
Imaging
- Abdominal X-ray for suspected obstruction, pseudo-obstruction or toxic megacolon
- CT abdomen/pelvis for obstruction, mass or complication
- Anorectal manometry with balloon expulsion test for suspected defecatory dyssynergia
- Colonic transit study for refractory constipation
Management
A. Treat the cause first
- Cease or substitute the culprit drug; correct hypothyroidism, hypercalcaemia, hypokalaemia, diabetes
- Treat coeliac disease, IBD, infection, pancreatic insufficiency specifically
B. Constipation - stepwise
1. Fibre (soluble - psyllium) and fluid, physical activity, toileting routine
- Insoluble fibre worsens bloating in IBS and is useless in slow transit
2. Osmotic: macrogol first-line, or lactulose (more bloating)
3. Stimulant: bisacodyl, senna - safe for regular use; the old fear of "cathartic colon" is not supported
4. Prucalopride (5-HT4 agonist), linaclotide or lubiprostone for refractory disease
5. Opioid-induced: methylnaltrexone or naloxegol (peripheral mu antagonists) - and reduce the opioid
6. Suppositories and enemas for faecal impaction; disimpact before starting oral agents
- Defecatory dyssynergia -> pelvic floor biofeedback, not more laxatives
C. Diarrhoea
- Rehydration and electrolyte correction
- Loperamide titrated to effect; *avoid antimotility agents in bloody diarrhoea, suspected C. difficile, or severe colitis - toxic megacolon risk*
- Bile acid diarrhoea: cholestyramine - often diagnostic as well as therapeutic
- Low-FODMAP diet with dietitian supervision and structured reintroduction
- Cholestyramine, ondansetron, or a low-dose TCA in functional diarrhoea
D. Pseudo-obstruction
- Correct electrolytes, cease opioids and anticholinergics, NG decompression, mobilise
- Neostigmine for acute colonic pseudo-obstruction not responding to conservative measures (monitored setting - bradycardia; have atropine ready)
- Colonoscopic decompression; surgery only for perforation or ischaemia
E. Functional disorders
- Positive diagnosis, clearly explained - a confident diagnosis is itself therapeutic
- Diet, exercise, sleep; gut-directed hypnotherapy or CBT
- Neuromodulators: low-dose TCA (diarrhoea-predominant), SSRI (constipation-predominant)
Traps
- Overflow diarrhoea from faecal impaction is the classic misdiagnosis - do the rectal examination**
- A new change in bowel habit over 50 is colorectal cancer until colonoscopy says otherwise, however typical the IBS story
- IBS should never be diagnosed for the first time in an older adult without excluding organic disease
- Nocturnal diarrhoea, weight loss and blood are never functional
- Iron deficiency anaemia with "IBS" is a missed cancer waiting to happen
- A normal colonoscopy does not exclude microscopic colitis - biopsies must be taken
- Coeliac serology is falsely negative on a gluten-free diet and in IgA deficiency - check total IgA
- Faecal calprotectin cannot distinguish IBD from colorectal cancer - it is not a cancer rule-out
- Laxatives will not fix pelvic floor dyssynergia; escalating them causes harm
- Antimotility agents in C. difficile or severe colitis precipitate toxic megacolon
- In a child with constipation, a perianal fistula or failure to thrive means this is not functional constipation
Talk track
- "I would start by clarifying what the patient actually means - frequency, consistency using the Bristol chart, urgency, and whether there is a sense of incomplete evacuation."
- "Then I would screen for red flags: rectal bleeding, weight loss, iron deficiency, a mass, and a new persistent change in bowel habit over the age of about fifty. Any of those means colonoscopy."
- "I would take a careful drug history, because so much of this is iatrogenic - opioids, metformin, magnesium, recent antibiotics."
- "My baseline tests would be a full blood count, iron studies, inflammatory markers, thyroid function, calcium, and coeliac serology with a total IgA. Faecal calprotectin helps me separate inflammatory bowel disease from irritable bowel syndrome."
- "A rectal examination is mandatory - it identifies impaction with overflow, perianal Crohn disease, and dyssynergic defecation, all of which change management completely."
- "If everything is negative and the Rome criteria are met, I would make a positive diagnosis of IBS rather than a diagnosis of exclusion, explain it confidently, and start with dietary modification and a neuromodulator if needed."
8 of 8 sections written · drafted 2026-09-08