Dr Kola Tytler MBBS CertHE MBA MRCGPClinical Lead • iatroX
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 NICE CKS,NICE CG61.
Key Differentiating Factors:
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 NICE CG61. 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 NICE CG61. Other supporting features can include lethargy, nausea, backache, and bladder symptoms NICE CG61. Importantly, IBS pain typically varies in site, unlike cancer-related pain which usually has a fixed site NICE CG61.
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 NICE CKS. Antibody testing for coeliac disease (IgA and IgA tissue transglutaminase or IgA endomysial antibody) should also be undertaken NICE CKS,NICE CG61. 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 NICE CKS.
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 NICE CKS. Emergency hospital admission is required if the person is systemically unwell with bloody diarrhoea, fever, tachycardia, or hypotension NICE CKS. Specialist investigations for IBD may include colonoscopy with histology, MRI of the small bowel, or CT scans NICE CKS.
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 NICE CKS,NICE CG61. These include:
- Unexplained weight loss NICE CKS.
- Unexplained rectal bleeding or persistent blood in the stool NICE CKS.
- An abdominal or rectal mass NICE CKS.
- Severe abdominal pain NICE CKS.
- Iron deficiency anaemia NICE CKS.
- Raised inflammatory markers such as Erythrocyte Sedimentation Rate (ESR) or C-reactive protein (CRP) NICE CKS,NICE CG61.
- Nocturnal or continuous diarrhoea NICE CKS.
- Fever, tachycardia, hypotension, or dehydration NICE CKS.
- A family history of inflammatory bowel disease NICE CKS.
- Rashes like pyoderma gangrenosum or erythema nodosum NICE CKS.
- Signs and symptoms of cancer, such as a positive quantitative faecal immunochemical test (FIT) NICE CKS,NICE CG61.
- 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 NICE CKS. 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 NICE CKS.
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 NICE CG61. 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 NICE CG61. Other supporting features can include lethargy, nausea, backache, and bladder symptoms NICE CG61. Importantly, IBS pain typically varies in site, unlike cancer-related pain which usually has a fixed site NICE CG61.
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 NICE CKS. Antibody testing for coeliac disease (IgA and IgA tissue transglutaminase or IgA endomysial antibody) should also be undertaken NICE CKS,NICE CG61. 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 NICE CKS.
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 NICE CKS. Emergency hospital admission is required if the person is systemically unwell with bloody diarrhoea, fever, tachycardia, or hypotension NICE CKS. Specialist investigations for IBD may include colonoscopy with histology, MRI of the small bowel, or CT scans NICE CKS.