What tests are used to diagnose Irritable Bowel Syndrome (IBS)?

Diagnostic Tests for Irritable Bowel Syndrome (IBS)

IBS is primarily diagnosed using symptom-based criteria (Rome criteria) and excluding other conditions through a limited set of targeted tests, rather than through extensive diagnostic testing. 1

Symptom-Based Diagnosis

• IBS diagnosis should be based on positive identification of symptoms using the Rome criteria, which requires abdominal pain for at least 12 weeks in the preceding 12 months with at least two of three features: pain relieved by defecation, onset associated with change in stool frequency, or onset associated with change in stool form 1
• Supporting symptoms include abnormal stool frequency, abnormal stool form, abnormal stool passage, mucus passage, and bloating/abdominal distention 1
• The diagnosis always presumes the absence of structural or biochemical explanations for symptoms 1

Basic Initial Testing

• Complete blood count (CBC) and C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR) should be performed in all patients with suspected IBS 1
• Coeliac serology (anti-endomysial antibodies) should be checked in all patients with IBS symptoms 1
• Stool testing for occult blood (Hemoccult) is recommended for screening purposes 1
• Faecal calprotectin should be tested in patients with diarrhoea under age 45 to exclude inflammatory bowel disease 1

Additional Testing Based on Clinical Presentation

For All Patients:

• Physical examination including digital rectal examination is essential 2
• Alarm features requiring further investigation include: unintentional weight loss, rectal bleeding, family history of colorectal cancer, or recent change in bowel habits 3, 2

Age-Based Testing:

• Patients over 50 years old or with a family history of colorectal cancer should undergo colonoscopy regardless of symptom pattern 1
• Young patients (<45 years) with typical IBS symptoms and no alarm features may be safely given a working diagnosis without further testing 1

Symptom-Specific Testing:

For IBS with diarrhea (IBS-D):

• In patients with atypical features such as nocturnal diarrhea or prior cholecystectomy, consider 23-seleno-25-homotaurocholic acid (SeHCAT) scanning or serum 7α-hydroxy-4-cholesten-3-one to exclude bile acid diarrhea 1
• Consider lactose/dextrose H2 breath test in patients who regularly consume dairy products, especially those from high-risk ethnic groups 1
• Consider stool microscopy for ova, cysts, parasites in patients with persistent diarrhea 1
• Colonoscopy with biopsies should be considered in patients with severe diarrhea to exclude microscopic colitis 1

For IBS with constipation (IBS-C):

• For persistent constipation symptoms, consider tests for slow colonic transit (whole gut transit test) 1
• In patients with symptoms suggestive of defecatory disorders, consider anorectal physiology tests or defecating proctography 1

Tests That Are NOT Recommended

• There is no role for colonoscopy in typical IBS without alarm features or age >50 years 1
• Ultrasound is not recommended as it often detects incidental findings unrelated to symptoms 1
• Hydrogen breath testing for small intestinal bacterial overgrowth is not recommended in patients with typical IBS symptoms 1
• Testing for exocrine pancreatic insufficiency is not indicated in typical IBS 1

Common Pitfalls to Avoid

• Overinvestigation with unnecessary tests increases healthcare costs and patient anxiety without improving outcomes 4, 2
• Failure to recognize alarm symptoms may lead to missed diagnoses of organic disease 3
• Relying solely on patient reports of food intolerances without objective testing may lead to unnecessary dietary restrictions 1
• Performing colonoscopy in young patients with typical IBS symptoms and no alarm features is not cost-effective 1, 3

Diagnostic Algorithm

1. Apply Rome criteria to identify IBS symptoms
2. Check for alarm features (weight loss, rectal bleeding, family history of colorectal cancer)
3. Perform basic testing: CBC, CRP/ESR, coeliac serology, and fecal calprotectin if diarrhea predominant
4. If basic tests are normal and no alarm features are present, make a positive diagnosis of IBS
5. For patients >50 years, add colonoscopy regardless of symptom pattern
6. For persistent symptoms despite treatment, consider symptom-specific testing as outlined above 1, 2

Remember that once the diagnosis of IBS is established, the incidence of new significant diagnoses is extremely low, with long-term follow-up studies showing minimal missed organic disease 1.