AKT · Gastroenterology/Colon & rectum

Irritable bowel syndrome

Functional gut–brain disorder (no structural pathology)

Overview

A common functional bowel disorder — abdominal pain related to defecation with altered bowel habit (IBS-D/C/M), bloating, and NO structural/biochemical abnormality. A positive clinical diagnosis (Rome/NICE criteria) after excluding red flags and key mimics (coeliac, IBD). Managed with diet, lifestyle and symptom-targeted drugs; reassurance is central.

Recognise

Abdominal pain/discomfort RELATED TO DEFECATION (relieved or triggered), with altered stool frequency/form and bloating, for ≥6 months
Subtypes: IBS-D (diarrhoea), IBS-C (constipation), IBS-M (mixed); worse with stress; mucus PR but NO blood
NO weight loss, rectal bleeding, nocturnal symptoms, or anaemia (those are red flags)

Red flags

Red flags → investigate, NOT IBS: rectal bleeding, weight loss, anaemia, nocturnal symptoms, family history of bowel/ovarian cancer, age ≥50 with new symptoms, abdominal/rectal mass
Raised faecal calprotectin → suspect IBD

Differentials & how to tell them apart

Coeliac diseasepositive tTG, weight loss/anaemia

Inflammatory bowel diseaseblood/mucus, raised calprotectin/CRP, nocturnal/weight loss — red flags

Colorectal cancerage, bleeding, weight loss, anaemia — 2-week-wait

Bile acid malabsorptionwatery diarrhoea responding to colestyramine; SeHCAT

Investigations

Positive clinical diagnosis after excluding mimics: FBC, CRP/ESR, COELIAC SEROLOGY, faecal CALPROTECTIN (excludes IBD); CA125 in women with relevant symptoms. Colonoscopy only if red flags.

Management

Diet/lifestyle + symptom-targeted drugs (antispasmodic / antidiarrhoeal / laxative)

1Make a POSITIVE diagnosis after excluding mimics (bloods, coeliac serology, faecal calprotectin). Diet/lifestyle advice (regular meals, fibre, low-FODMAP with a dietitian); symptom-targeted drugs.Gate: Any RED FLAG (rectal bleeding, weight loss, anaemia, nocturnal symptoms, age ≥50 new change, mass, family history) means investigate for organic disease — it is NOT IBS; a raised faecal calprotectin points to IBD
2Refractory pain → low-dose amitriptyline/SSRI; IBS-C → linaclotide; IBS-D → loperamide; psychological therapies (CBT/gut-directed hypnotherapy).

Diet/lifestyle (regular meals, fibre adjustment, low-FODMAP with dietitian) — first-line; limit caffeine/alcohol/fizzy drinks

Antispasmodic (mebeverine/hyoscine) — for pain/cramps

Laxative (not lactulose) for IBS-C; loperamide for IBS-D — symptom-targeted; linaclotide for refractory IBS-C

Low-dose tricyclic (amitriptyline) / SSRI — second-line for pain (gut–brain modulation)

Key points

Pain related to defecation + altered habit + bloating + NO red flags = IBS (a positive diagnosis, not just exclusion). Faecal calprotectin and coeliac serology are the key 'rule-outs'. Blood/weight loss/nocturnal symptoms are never IBS.

Monitor & prognosis

Symptom response; reassess if red flags emerge.

Chronic-relapsing but benign.

Source: NICE CG61; CKS IBS