Reasoning GP · Algorithm · 1-page summary
Chronic diarrhoea in adults
For: adults ≥18 with loose or watery stool (Bristol 5–7) persisting ≥4 weeks, a change from baseline. Not for: children or acute diarrhoea (use their own pathways). Urgency or incontinence with normal stools is not diarrhoea.
Core rule: screen every new presentation for red flags first. Any NG12 colorectal feature → FIT, and a negative FIT never overrides a red flag. IBS-D is a positive diagnosis, never just "all tests normal".
0Stop — call 999 now
- Severe dehydration with shock, or profuse bloody diarrhoea with collapse → 999. Toxic megacolon (IBD with distension, fever, peritonism or abdominal rigidity) or septic shock from an enteric source → 999
- Do not give loperamide or other antimotility drugs with bloody diarrhoea, fever, suspected C. difficile, severe IBD, distension or obstruction.
1Triage by stool pattern — mechanism drives the tests
| Pattern | Think of |
|---|---|
| Blood, mucus, fever | IBD, infection, C. difficile, cancer |
| Fatty, pale, offensive | Coeliac, pancreatic insufficiency |
| Watery, nocturnal | Bile acid, microscopic colitis, Giardia |
| Stops with fasting | Lactose, sorbitol, magnesium, laxatives |
| Pain eased by defaecation | IBS-D (no red flags, tests reassuring) |
2Screen — one row is enough
- Rectal bleeding, persistent change in bowel habit, iron-deficiency anaemia or abdominal mass (any age) → FIT; ≥10 µg Hb/g → Suspected cancer pathway. Rectal or anal mass → refer without FIT.
- Weight loss ≥5% over 6 months → FIT; 55+ with upper abdominal pain, reflux or dyspepsia → Urgent direct-access OGD; 60+ with weight loss and diarrhoea → consider urgent direct-access CT (pancreas).
- Profuse rectal bleeding or symptomatic anaemia → Same-day (999 if haemodynamically compromised)
- Fever + bloody diarrhoea → stool culture; Same-day, 999 if septic or shocked. Suspected E. coli O157: no loperamide or antibiotic (HUS).
- Severe IBD flare (≥6 bloody stools/day plus T >37.8°C, pulse >90, Hb <105 g/L or ESR >30), C. difficile with systemic upset, unable to maintain oral hydration → Same-day
- Immunocompromised (HIV, transplant, chemotherapy, biologics) → Urgent specialist input
3Assess
☐ Weight, pallor, skin, eyes, joints, thyroid; abdomen and PR (mass, perianal disease) ☐ Full drug review incl. OTC (metformin, PPI, SSRI, magnesium, laxatives) ☐ FBC, CRP/ESR, tTG IgA + total IgA (eating gluten in >1 meal/day for ≥6 weeks); U&E, albumin, LFT, TFT, HbA1c if indicated ☐ FIT if any NG12 feature ☐ Calprotectin only if IBD vs functional and cancer not suspected (local pathway) ☐ Faecal elastase if steatorrhoea ☐ Stool culture/PCR or C. difficile when indicated. Not here: routine CT, breath tests, colonoscopy first-line without red flags or raised calprotectin.
4Classify — only if the screen is negative
- IBD — blood/mucus, raised CRP or calprotectin, perianal disease → Urgent gastroenterology.
- Coeliac — positive tTG → Urgent gastroenterology for duodenal biopsy; no gluten-free diet before biopsy.
- Microscopic colitis — watery, woman >50, NSAID/PPI → Urgent gastroenterology; colonoscopy with biopsies.
- Malabsorption — steatorrhoea, low faecal elastase → Urgent gastroenterology. Bile acid — post-cholecystectomy or ileal resection → SeHCAT via gastroenterology.
- Drug or diet (metformin, sorbitol, lactose) → switch or exclude. IBS-D — recurrent pain related to defaecation, no red flags, normal FBC, CRP/ESR and coeliac serology.
- Uncertain — unexplained diarrhoea with a normal work-up → Routine gastroenterology; IBS needs positive features.
5Treat today — treat the cause; doses: NICE/SPC, BNF, local formulary
- IBS-D: antispasmodic (e.g. mebeverine 135 mg TDS) + loperamide, start low and review, only without infective or inflammatory red flags.
- C. difficile: stop the causative antibiotic if safe, review PPIs and laxatives; oral vancomycin 125 mg QDS × 10 days; fidaxomicin 200 mg BD × 10 days if ineffective or recurrence within 12 weeks. Specialist advice if older, frail, immunocompromised or severe.
- Drug-induced: review with the patient; metformin → consider modified-release.
- Lactose: lactose-free trial 4–6 weeks, keep calcium intake adequate.
6Refer routinely
- Gastroenterology: IBS-D not responding to primary care treatment, suspected bile acid malabsorption, SIBO, Giardia needing specialist oversight.
- Dietitian: coeliac (after biopsy), malabsorption, low-FODMAP for IBS-D when first-line advice fails.
- Review results at 2–4 weeks; coeliac annual review.
7Never reassure by
- A negative or unavailable FIT when concern remains (it misses upper GI, pancreas, ovary, IBD).
- A low calprotectin: it does not exclude cancer, coeliac, bile acid, pancreatic or microscopic colitis.
- Negative coeliac serology if gluten was restricted or total IgA is low.
- Nocturnal diarrhoea labelled as IBS.
- Rectal bleeding put down to haemorrhoids.
- An "established" IBS label when the pattern changes, especially at 50+.
8Safety-net — say it, then document it
999 “Call 999 for sudden severe bloody diarrhoea, collapse, a rigid tummy, or severe dehydration with confusion or drowsiness.” Same day: “Contact us today for any new bleeding from the bottom, a fever over 38°C, being unable to keep fluids down, marked weight loss, a new lump in the tummy, or symptoms getting worse despite treatment.”
At every follow-up: re-screen red flags; new weight loss, bleeding or systemic symptoms in "known IBS-D" → re-investigate.
✓Safe to leave primary care only if — all five, documented
No block 0–2 row applies, or FIT/referral done
Baseline bloods and coeliac serology on gluten
Full drug review done
Cause named, or uncertainty stated with a referral
Results review booked; safety-net given
Summary of the full Reasoning GP chronic diarrhoea algorithm, which holds the detail, examination, tests and evidence. Sources: NICE NG12 (updated April 2026), NG20, HTG320, HTG690, CG61, NG199; BSG chronic diarrhoea, coeliac, IBD and IBS guidelines. Clinical reference — follow local pathways (local guidance) and check doses: NICE/SPC, BNF.