01OverviewDefinition, clinical context and the essential points that orientate the chapter.
IBS is a chronic, relapsing disorder in which altered motility, visceral sensitivity, mucosal signalling, microbiota, diet and central processing can contribute in different proportions. The diagnosis is clinically meaningful: abdominal pain is associated with defaecation or a change in stool frequency or form, accompanied by symptoms such as urgency, incomplete evacuation, mucus and bloating. Establish duration, Bristol stool form, pain-stool relationship, nocturnal symptoms and impact. Ask sensitively about faecal incontinence, pelvic-floor symptoms, eating restriction, anxiety and previous gastroenteritis. IBS-D, IBS-C and IBS-M are management phenotypes rather than fixed identities. Symptoms may overlap functional dyspepsia, reflux, migraine, fibromyalgia and pelvic pain without making any of them unreal.
Proportionate investigation protects against both missed organic disease and harmful overtesting. Review family history, bleeding, anaemia, weight trajectory, fever, nocturnal diarrhoea, older-age onset, masses, recent antibiotics, travel, medicines and gynaecological features. A typical stable adult generally needs full blood count, CRP or ESR and coeliac antibodies. Faecal calprotectin helps distinguish inflammatory bowel disease from a functional disorder when cancer is not suspected and the local pathway regards it as appropriate. FIT, endoscopy or imaging responds to cancer criteria or specific discordant findings, not patient anxiety alone. Normal routine tests do not mean no illness; they complete the evidence for a positive diagnosis.
Management is collaborative experimentation with measurable goals. Explain the mechanism, agree which symptom matters most and change one or two variables at a time. Regular eating, adequate non-caffeinated fluid, exercise and adjustment of soluble fibre can be more sustainable than a dramatic diet. Medicines target stool form or pain and need review for benefit and adverse effects. Dietitian-led low-FODMAP treatment includes restriction, systematic reintroduction and personalisation to limit nutritional and social harm. Refractory symptoms call for a diagnostic reset and gut-directed CBT, hypnotherapy or another accessible psychological treatment where indicated. Follow-up should record function, not just pain, and reopen investigation when warning features emerge.
Key points
- IBS is a disorder of gut-brain interaction characterised by recurrent abdominal pain linked to defaecation or altered stool frequency or form, not a diagnosis defined by bloating alone.
- Classify the current pattern as constipation-predominant, diarrhoea-predominant, mixed or unclassified because treatment follows the dominant stool problem and can change over time.
- Make a positive diagnosis when the symptom pattern, examination and limited tests fit; repeated normal scans and colonoscopies can reinforce uncertainty rather than improve care.
- Check full blood count, inflammatory markers and coeliac serology in a typical presentation, adding faecal calprotectin or cancer-pathway testing only when age and features justify them.
- General measures include regular meals, hydration, physical activity and soluble rather than insoluble fibre; bran frequently worsens bloating and pain.
- A low-FODMAP or other exclusion diet should be structured by a clinician with dietary expertise and followed by reintroduction, not continued as an unsupervised ever-shrinking food list.
- Use antispasmodics for pain, loperamide for diarrhoea and suitable laxatives for constipation, titrating towards a soft formed stool rather than complete symptom abolition.
- Low-dose tricyclic treatment acts as a gut-brain neuromodulator in persistent pain; explain the rationale so the patient does not hear that symptoms are imagined.
02AetiologyUnderlying causes, associations and risk factors, with why each one matters.
Gut-brain susceptibility
Genetic, developmental and psychosocial factors shape visceral sensation and central pain regulation, creating vulnerability without a single diagnostic lesion.
Post-infective and microbial factors
Some IBS begins after gastroenteritis, with persistent immune, barrier and microbiome changes despite clearance of the original pathogen.
Dietary and stress modifiers
Fermentable carbohydrates, irregular meals, sleep disruption and stress can amplify symptoms, but response to a trigger does not establish an allergy or intolerance.
03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.
- 1Visceral hypersensitivity
Ordinary gas, stool or bowel contraction generates exaggerated afferent signalling and pain because peripheral and central thresholds are altered.
- 2Motility and secretion change
Variable transit and fluid handling produce constipation, diarrhoea or a mixed pattern that can shift over time.
- 3Bidirectional brain-gut reinforcement
Pain, autonomic arousal, attention and avoidance feed back on motility and sensation, sustaining genuine symptoms without structural inflammation.
04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.
Recurrent abdominal pain associated with loose stools, urgency or defaecation, with a stable examination and no inflammatory, malignant or malabsorptive signal, supports IBS-D after proportionate testing.
Pain, bloating, hard or infrequent stools and incomplete evacuation fit IBS-C. Marked straining, digital manoeuvres or a sense of blockage should also raise a defaecatory pelvic-floor disorder.
A palpable mass, iron-deficiency anaemia, rectal bleeding or an age- and symptom-qualified bowel change must be assessed using current NG12 and FIT pathways rather than explained by longstanding IBS.
Post-cholecystectomy watery urgency, ileal disease or exposure to metformin, magnesium, laxatives or other diarrhoeal medicines can produce a treatable phenotype that should not be relabelled automatically.
05InvestigationsWhat to request, why it matters and how to interpret it.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Full blood count and inflammatory markersFirst step - Why
- Look for anaemia and objective inflammation that would make uncomplicated IBS less likely.
- Interpretation and limitations
- Normal results support a positive diagnosis in a compatible presentation. Iron deficiency, thrombocytosis or a raised CRP or ESR demands explanation rather than escalation of symptomatic IBS treatment.
- 02
Coeliac serology - Why
- Exclude coeliac disease, which can resemble any IBS stool phenotype and has a specific treatment.
- Interpretation and limitations
- Request total IgA with IgA tissue transglutaminase while gluten remains in the diet. Positive or equivocal results and IgA deficiency follow the appropriate confirmation pathway.
- 03
Faecal calprotectin - Why
- Differentiate inflammatory bowel disease from a functional bowel disorder in selected adults when cancer is not suspected.
- Interpretation and limitations
- Interpret with the local threshold and repeat pathway. Infection, NSAIDs and other inflammation can raise it; a low result is reassuring for IBD but does not exclude coeliac disease, bile acid diarrhoea or microscopic colitis.
- 04
FIT and suspected-cancer assessment - Why
- Triage colorectal cancer risk when the symptom combination meets current NICE or local pathway criteria.
- Interpretation and limitations
- A result is acted on within its pathway and does not independently overrule a mass, significant anaemia or clinical deterioration. IBS should not delay urgent referral when criteria are met.
- 05
Stool microbiology - Why
- Investigate diarrhoea with travel, outbreak, antibiotic, immunosuppression or acute infectious clues rather than using it routinely.
- Interpretation and limitations
- A positive organism redirects infection management. Persistent watery symptoms after negative targeted testing still require phenotype-specific assessment, including microscopic colitis or bile acid diarrhoea where appropriate.
- 06
Colonoscopy - Why
- Answer a defined structural, inflammatory or cancer question when red flags, abnormal tests or an atypical course justify endoscopy.
- Interpretation and limitations
- A normal examination is not required to diagnose every typical case. If chronic watery diarrhoea is the question, ensure colonic biopsies are taken because normal mucosa does not exclude microscopic colitis.
06Differential diagnosisRealistic alternatives and the features that help distinguish them.
Inflammatory bowel disease
Bleeding, nocturnal symptoms, weight loss, objective inflammation or raised calprotectin favours IBD over a typical IBS phenotype.
Coeliac disease
Positive serology obtained while eating gluten and compatible enteropathy identify coeliac disease, which may otherwise mimic IBS with diarrhoea or bloating.
Cancer, microscopic or bile-acid disease
Older-onset change, anaemia, mass or bleeding requires cancer assessment; nocturnal watery stool or post-ileal diarrhoea directs colonic biopsies or bile-acid testing.
Additional chapter-specific clues
Blood mixed with stool, night-time diarrhoea, fever, raised inflammatory markers, perianal disease or progressive weight loss is discordant and requires assessment for IBD, infection or microscopic colitis.
Iron or folate deficiency, weight loss, mouth ulcers, dermatitis herpetiformis, autoimmune disease or a family history supports coeliac testing while the patient continues eating gluten.
07ManagementImmediate care, first-line treatment, alternatives and escalation.
01Positive diagnosisConfirm pattern and exclude proportionatelyFirst stepRecurrent abdominal pain and altered bowel habit suggest a disorder of gut-brain interaction.+
- 1Map pain to defaecation and stool change, record Bristol form and duration, examine the abdomen and rectum when clinically indicated, and ask about warning features.
- 2Request full blood count, inflammatory markers and coeliac serology, adding calprotectin, infection testing, FIT or endoscopy only for a defined indication.
- 3Name the current IBS subtype, explain gut-brain mechanisms in neutral language and acknowledge that a positive diagnosis does not mean symptoms are psychological invention.
- 4Agree one functional goal, one treatment target and explicit triggers for reassessment, documenting why further investigation is or is not needed.
02First-line careTreat the dominant phenotypeFirst lineTypical IBS is established without a current red flag or untreated organic mimic.+
- 1Begin regular meals, adequate fluid, graded physical activity and a review of caffeine, alcohol, fizzy drinks, sorbitol and fibre, avoiding a universal restrictive checklist.
- 2For constipation favour soluble fibre and an appropriate laxative; for diarrhoea consider loperamide; for episodic pain consider an antispasmodic and titrate by response.
- 3Use a symptom and food record for a limited period, altering one intervention at a time so benefit and harm can be attributed.
- 4Review after an agreed interval for stool form, pain, urgency, sleep, participation and adverse effects, then stop ineffective measures rather than accumulating treatments.
03Persistent IBSEscalate without abandoning safetyFirst lineEscalationSymptoms remain disruptive after a credible first-line programme.+
- 1Recheck the diagnosis, medicine exposure, adherence, pelvic-floor features, eating restriction and any newly emerged bleeding, anaemia, weight loss or night-time symptoms.
- 2Refer for dietitian-led exclusion and reintroduction if dietary treatment is chosen, protecting nutritional adequacy and avoiding indefinite blanket restriction.
- 3Consider low-dose TCA neuromodulation for persistent pain after stool-targeted measures, reviewing early for anticholinergic, cardiac, mood and overdose risks.
- 4Offer accessible gut-directed psychological therapy for refractory symptoms and coordinate gastroenterology review when the phenotype, tests or treatment response remain discordant.
Key medicines and prescribing safety4 treatments · regimens, roles and cautions+
Loperamide
Use the current formulary starting regimen for IBS-D and let the patient titrate cautiously towards Bristol type 4 rather than a fixed maximal schedule.Avoid during acute bloody diarrhoea, fever, suspected ileus or severe inflammatory colitis. Excess dosing can cause constipation and serious cardiac toxicity; review continued need and response.
Soluble fibre or laxative
Introduce ispaghula or another suitable constipation treatment gradually with fluid, then adjust to a soft formed stool using the local formulary sequence.Escalation may increase gas or pain; avoid bulk agents in suspected obstruction. NICE discourages lactulose in IBS because it can worsen bloating, and persistent outlet symptoms need pelvic-floor assessment.
Low-dose tricyclic antidepressant
NICE describes 5 to 10 mg amitriptyline-equivalent at night initially, reviewed regularly and usually not increased beyond 30 mg for IBS symptoms.Discuss unlicensed symptom use where applicable, drowsiness, constipation, dry mouth, urinary retention, glaucoma, cardiac conduction, falls and overdose risk. Review at four weeks and avoid abrupt unsupervised changes.
Linaclotide
Use the licensed and locally formulary-approved daily regimen only after different laxative classes fail in longstanding IBS-C, with review at three months.Diarrhoea can be important; exclude mechanical obstruction and review hydration. NICE eligibility includes at least twelve months of constipation and failure of maximum tolerated prior laxatives.
08ComplicationsImportant consequences, why they occur and why they matter clinically.
Restrictive diet and deficiency
Unsupervised exclusion diets can reduce fibre, calcium and overall diversity, worsen constipation and create anxiety around food.
Medicine and investigation burden
Repeated low-yield testing or escalating laxative, antidiarrhoeal and analgesic use can cause adverse effects and undermine a confident diagnosis.
Quality-of-life impairment
Pain, urgency and unpredictable stool disrupt work, travel, intimacy and mental health despite IBS not causing inflammation, bleeding or cancer.
09Monitoring and follow-upTreatment response, safety checks and longer-term review.
- Follow abdominal pain days, Bristol stool form, urgency, straining, incontinence and the patient's chosen activity goal rather than a single global symptom score.
- Record weight and diet breadth when restriction is used, involving a registered dietitian if nutritional adequacy, avoidant eating or social isolation becomes a concern.
- Review any new TCA after four weeks for benefit, anticholinergic effects, sedation, mood and safety, then at appropriate six- to twelve-month intervals if continued.
- Check laxative or loperamide titration for constipation-diarrhoea cycling, stopping treatments that simply push the patient between extremes.
- At every meaningful review, ask about rectal bleeding, anaemia symptoms, night-time diarrhoea, fever, mass or unintentional weight loss and reopen investigation if present.
10Special situationsVariants, exceptions and circumstances that change the usual approach.
Positive is not dismissive
A symptom pattern plus targeted normal tests can positively identify IBS. Explaining the evidence and mechanism often reduces more anxiety than saying that every investigation is normal.
Subtype follows the stool
An individual can move from IBS-C to IBS-M or IBS-D as disease, diet and medicines change. Reclassify the current phenotype before simply repeating an old prescription.
Fibre has personalities
Insoluble bran increases stool bulk and fermentation and may intensify pain, whereas slowly introduced soluble fibre can improve global symptoms. Asking only whether fibre was tried hides this distinction.
Calprotectin has a job description
It helps separate inflammatory bowel disease from functional symptoms in a defined population. It is not a generic bowel-cancer test and may be normal in microscopic colitis.
Reintroduction completes diet therapy
The purpose of low-FODMAP restriction is to identify tolerable triggers, not maintain maximal exclusion. Reintroduction widens nutrition, cost and social choice while preserving symptom benefit.
11Common pitfallsFrequent interpretation and management errors.
- 01
Diagnosing IBS from bloating alone without the defining abdominal-pain and bowel-habit relationship.
- 02
Repeating colonoscopy for a stable typical phenotype while omitting coeliac serology.
- 03
Using faecal calprotectin as a colorectal cancer rule-out test or as proof of microscopic colitis.
- 04
Advising more bran to every constipated patient despite worsening pain and distension.
- 05
Allowing an unsupervised exclusion diet to become nutritionally inadequate and socially disabling.
- 06
Attributing new rectal bleeding or iron deficiency to a historic IBS diagnosis.