PassFinals
Gastroenterology & Nutrition

Irritable Bowel Syndrome

A chronic disorder of gut-brain interaction diagnosed positively from abdominal pain related to defaecation or altered stool pattern, once relevant red flags and targeted tests have been addressed.

In a nutshell

IBS is a positive diagnosis of a chronic gut-brain interaction disorder: abdominal pain is related to defaecation or altered stool pattern, with associated bowel symptoms and no alarm features. Use targeted blood and inflammatory-marker testing, then treat the dominant symptom with explanation, diet and stepped therapy.

Classic presentation

An adult with recurrent abdominal pain better after defaecation, bloating and diarrhoea, constipation or alternating stool form, often triggered by meals or stress, with no red flags and appropriate screening tests that do not suggest coeliac disease, anaemia or bowel inflammation.

Key points

  • Diagnose IBS positively from the symptom pattern; it is not a reason for endless normal tests.
  • The core pattern is abdominal pain related to defaecation or altered stool frequency/form plus at least 2 associated bowel symptoms.
  • Initial tests include FBC, CRP or ESR/plasma viscosity and coeliac serology; faecal calprotectin helps assess inflammatory causes, especially with diarrhoea or mixed symptoms.
  • Start with regular meals, activity, relaxation and tailored fibre advice; use specialist dietetic support for a structured low-FODMAP trial and reintroduction.
  • Treat the dominant symptom: antispasmodic for pain, laxative but not lactulose for constipation, and loperamide first choice for diarrhoea.
  • Red flags or an atypical/progressive course require investigation or referral rather than stronger IBS medication.

First-line investigation

Positive symptom assessment with examination, FBC, CRP or ESR/plasma viscosity and coeliac serology; add faecal calprotectin when inflammatory bowel disease is a concern.

Management

Check that this is safe to manage as IBS

  • Assess for bleeding, weight loss, anaemia, inflammatory markers, masses, suspected-cancer features and other discordant or progressive symptoms before reassuring.1,4,2

Confirm the positive symptom pattern

  • Use the pain-and-stool relationship, associated bowel symptoms and targeted tests to make a positive diagnosis and explain gut-brain interaction.1,3

Start lifestyle, diet and symptom-targeted treatment

  • Give regular-meal, activity, fluid, relaxation and fibre advice, then choose antispasmodic, constipation or diarrhoea treatment according to the dominant symptom.1,5,8

Use dietetic and gut-brain support

  • Use an expert-led low-FODMAP approach with reintroduction when general advice has not helped; consider psychological support for persistent symptom burden.1,7

Escalate refractory or atypical symptoms

  • Consider a low-dose TCA after first-line symptom medicines have failed, and seek specialist assessment when symptoms remain refractory, atypical or diagnostically uncertain.1,8

Review response and safety-net

  • Agree follow-up and an annual review; investigate or refer if red flags emerge or the course no longer fits IBS.1,4

Exam traps

  • IBS is not diagnosed by excluding every disease: use the positive symptom pattern and targeted tests.
  • A raised inflammatory marker, anaemia, rectal bleeding, weight loss or mass should redirect the pathway.
  • Do not recommend lactulose for IBS-related constipation; consider linaclotide only after the NICE criteria are met.
  • Loperamide is the first-choice antimotility agent for IBS-related diarrhoea; titrate to stool consistency and reassess atypical diarrhoea.
  • Low-FODMAP advice should be specialist-led and followed by reintroduction, not an indefinite unsupervised exclusion diet.

Illustrations

The gut-brain axis in IBSDiagram illustrating bidirectional signalling between the enteric nervous system, gut microbiome and central nervous system, with visceral sensitivity amplifying normal gut signals.PassFinals · original
Mechanism of FODMAP-triggered bloatingDiagram showing poorly absorbed fermentable carbohydrates drawing water osmotically and being fermented into gas, provoking symptoms in a sensitive gut.PassFinals · original
IBS symptom subtypesIllustration contrasting diarrhoea-predominant, constipation-predominant and mixed IBS as different expressions of altered bowel habit.PassFinals · original

Key sources

  1. NICE, Irritable bowel syndrome in adults: diagnosis and management (CG61)
  2. NHS, Getting diagnosed with IBS
  3. NICE, Irritable bowel syndrome in adults quality standard: excluding inflammatory causes (QS114)
  4. NICE, Suspected cancer: recognition and referral (NG12)
  5. NHS, Diet, lifestyle and medicines for IBS
  6. NICE, Faecal calprotectin diagnostic tests for inflammatory diseases of the bowel (HTG320)
  7. British Dietetic Association, IBS and diet
  8. British National Formulary, online prescribing information

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.