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How do I differentiate between inflammatory bowel disease and irritable bowel

Guideline-aligned answer with reasoning, red flags and references. Clinically reviewed by Dr Kola Tytler MBBS CertHE MBA MSt MRCGP.

Posted: 16 August 2025Updated: 16 August 2025 Guideline-Aligned (High Confidence) Clinically Reviewed
To differentiate between inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS) in a patient presenting with chronic diarrhoea, the assessment primarily focuses on identifying "red flag" indicators and utilising specific diagnostic tests ,. Key Differentiating Factors:
  • Red Flag Indicators: The presence of certain symptoms or features strongly suggests an organic cause like IBD rather than IBS, necessitating further investigation and potential urgent referral to secondary care ,. These include:
    • Unexplained weight loss .
    • Unexplained rectal bleeding or persistent blood in the stool .
    • An abdominal or rectal mass .
    • Severe abdominal pain .
    • Iron deficiency anaemia .
    • Raised inflammatory markers such as Erythrocyte Sedimentation Rate (ESR) or C-reactive protein (CRP) ,.
    • Nocturnal or continuous diarrhoea .
    • Fever, tachycardia, hypotension, or dehydration .
    • A family history of inflammatory bowel disease .
    • Rashes like pyoderma gangrenosum or erythema nodosum .
    • Signs and symptoms of cancer, such as a positive quantitative faecal immunochemical test (FIT) ,.
  • Faecal Calprotectin Testing: This test is specifically used to help differentiate between IBS and IBD in people under the age of 40 years, particularly when specialist assessment is being considered and cancer is not suspected . It should not be used for people with new onset rectal bleeding or bloody diarrhoea, or when there is a need to rule out cancer .
Irritable Bowel Syndrome (IBS) Diagnosis: A diagnosis of IBS is considered if the patient has had abdominal pain or discomfort for at least 6 months that is either relieved by defaecation or associated with altered bowel frequency or stool form . This must be accompanied by at least two of the following symptoms: altered stool passage (straining, urgency, incomplete evacuation), abdominal bloating (distension, tension, or hardness), symptoms made worse by eating, or passage of mucus . Other supporting features can include lethargy, nausea, backache, and bladder symptoms . Importantly, IBS pain typically varies in site, unlike cancer-related pain which usually has a fixed site . Initial Investigations for Chronic Diarrhoea: For all people with chronic diarrhoea, initial blood tests should include a full blood count, urea and electrolytes, liver function tests (including albumin), calcium, vitamin B12 and red blood cell folate, iron status (ferritin), thyroid function tests, ESR, and CRP . Antibody testing for coeliac disease (IgA and IgA tissue transglutaminase or IgA endomysial antibody) should also be undertaken ,. Stool samples may be sent for routine microbiology, ova, cysts, and parasites if an infectious cause is suspected, or for Clostridioides difficile testing if there's a history of recent hospital admission, antibiotic, or proton pump inhibitor use . Referral for Suspected IBD: If Crohn's disease or another form of IBD is suspected, especially with red flag symptoms, an urgent referral to secondary care (gastroenterologist) is indicated for confirmation of diagnosis and specialist management . Emergency hospital admission is required if the person is systemically unwell with bloody diarrhoea, fever, tachycardia, or hypotension . Specialist investigations for IBD may include colonoscopy with histology, MRI of the small bowel, or CT scans .

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