IBS Work-Up, Diagnosis, and Initial Treatment
Diagnostic Approach
In adults under 45 years with typical IBS symptoms and no alarm features, make a positive clinical diagnosis using Rome criteria without extensive testing—this approach is safe, cost-effective, and supported by major gastroenterology societies. 1, 2
Rome Criteria for Diagnosis
Apply the Rome III criteria: recurrent abdominal pain or discomfort for at least 12 weeks (need not be consecutive) in the preceding 12 months, with at least two of the following three features: 1, 2
- Pain relieved by defecation
- Onset associated with change in stool frequency
- Onset associated with change in stool form (appearance)
Supportive Clinical Features
The diagnosis is more likely if the patient is: 1
- Female
- Age <45 years
- Symptom duration >2 years
- History of frequent visits for non-gastrointestinal symptoms (e.g., malaise, backache)
Mandatory Screening Tests (All Patients)
Perform these baseline investigations in every patient with suspected IBS to exclude organic disease: 2, 3
| Test | Purpose | Reference |
|---|---|---|
| Complete blood count (CBC) | Exclude anemia and inflammatory changes | [2,3] |
| C-reactive protein (CRP) or ESR | Screen for inflammatory bowel disease (note: ~20% of active Crohn's patients have normal CRP, so normal result does not fully exclude IBD) | [2,3] |
| Celiac serology (IgA tissue transglutaminase + total IgA) | Detect celiac disease (>90% sensitivity); use IgG-based testing if IgA-deficient | [2,3] |
| Fecal calprotectin | Values <50 µg/g exclude IBD; >200–250 µg/g suggest IBD, especially in patients <45 with diarrhea | [2,3] |
| Stool test for Giardia | Identify treatable parasitic cause of chronic diarrhea | [2] |
| Fecal occult blood test | Screen for occult gastrointestinal bleeding | [2,3] |
Alarm Features Requiring Extended Work-Up
Any of the following mandates further investigation (typically colonoscopy and additional testing): 1, 2, 3
| Alarm Feature | Clinical Implication |
|---|---|
| Age ≥45 years at symptom onset | Higher risk of colorectal cancer; colonoscopy required |
| Unintentional weight loss | Suggests malignancy or inflammatory disease |
| Rectal bleeding or blood in stool | Requires endoscopic evaluation |
| Anemia on CBC | Indicates organic disease; excludes functional IBS |
| Nocturnal symptoms (pain or diarrhea waking patient from sleep) | Associated with organic pathology |
| Fever | Suggests systemic inflammation or infection |
| Family history of IBD or colorectal cancer | Increases pre-test probability of serious disease |
When to Perform Colonoscopy
Colonoscopy is indicated when: 1, 2, 3
- Age ≥45 years (or ≥50 years per some guidelines) at symptom onset
- Any alarm feature present (see table above)
- Family history of colorectal cancer or inflammatory bowel disease
- Atypical symptoms or short symptom duration
Colonoscopy is NOT indicated in patients <45 years with typical IBS symptoms and no alarm features—this approach avoids unnecessary procedures and cost. 2, 3
Endoscopic Technique
When colonoscopy or sigmoidoscopy is performed: 1
- Biopsy both abnormal-appearing and normal-appearing mucosa
- In patients with diarrhea, obtain biopsies to detect microscopic colitis even if mucosa appears normal
Additional Testing (Selective Use)
Consider these tests only in specific clinical contexts: 1, 2
| Test | Indication | Yield |
|---|---|---|
| Thyroid function tests | Unexplained diarrhea or constipation | 1–2% positive |
| Lactose breath test | Patients consuming >0.5 pint (280 mL) milk daily, especially high-risk ethnic groups | 8–25% positive depending on population |
| Bile acid diarrhea testing (SeHCAT scan or serum 7α-hydroxy-4-cholesten-3-one) | IBS-D not responding to initial therapy | Variable |
| Stool microscopy and laxative screen | Chronic diarrhea with atypical features | 1–2% positive each |
Tests NOT Recommended
Avoid these investigations in typical IBS without alarm features: 2, 3
- Abdominal ultrasound (detects incidental findings unrelated to symptoms)
- Hydrogen breath testing for small intestinal bacterial overgrowth (SIBO)
- Testing for exocrine pancreatic insufficiency
- Serologic tests marketed for IBS diagnosis (sensitivity <50%)
- Stool testing for ova and parasites (except Giardia), unless travel to or immigration from endemic areas
- Routine CRP or ESR alone to screen for IBD (conditional recommendation against)
Initial Management
1. Patient Education and Reassurance
Provide a positive diagnosis with detailed explanation—this is the cornerstone of management and improves outcomes: 1
- Explain that IBS is a chronic functional disorder with benign prognosis
- Emphasize that once functional diagnosis is established, the incidence of new organic diagnoses is extremely low
- Listen to patient concerns and address specific fears (e.g., cancer, serious disease)
- Avoid serial repetitive testing, which increases anxiety
2. Lifestyle and Dietary Modifications
First-line non-pharmacologic interventions: 1
- Identify and eliminate food triggers (common culprits: wheat, milk, coffee, potatoes, corn, onion, beef, oats, cheese, white wine)
- Consider lactose restriction only if patient consumes substantial lactose (>0.5 pint/280 mL milk daily) and has positive breath test
- Encourage regular exercise and adequate sleep
- Address lack of dietary fiber or fiber excess
3. Pharmacologic Treatment (Symptom-Based)
Tailor therapy to predominant bowel habit: 1, 4, 5
For IBS with Constipation (IBS-C):
- Soluble fiber (e.g., psyllium) as first-line
- Osmotic laxatives (e.g., polyethylene glycol)
- Linaclotide 290 mcg once daily for patients not responding to fiber/laxatives 6
For IBS with Diarrhea (IBS-D):
- Loperamide (opioid antidiarrheal) for diarrhea control
- Rifaximin 550 mg three times daily for 14 days (FDA-approved for IBS-D) 7
For Abdominal Pain:
- Antispasmodics (e.g., hyoscyamine, dicyclomine) for pain relief 1, 4
- Low-dose tricyclic antidepressants (e.g., amitriptyline 10–25 mg at bedtime) to normalize gastrointestinal motility and reduce visceral hypersensitivity 4, 5
4. Avoid These Medications
Do not prescribe opioid analgesics for IBS pain—they worsen constipation and are contraindicated in functional bowel disorders. 2
Follow-Up and Referral
Schedule review in 4–6 weeks to assess response to initial management. 2
Refer to Gastroenterology if:
- Symptoms persist despite optimized first-line therapy (3–6 weeks)
- Atypical or severe symptoms develop
- New alarm features emerge
- Patient develops significant anxiety requiring specialist reassurance
- Age reaches ≥45 years with ongoing symptoms
Common Pitfalls to Avoid
- Over-testing young patients with typical symptoms: Colonoscopy in patients <45 without alarm features is not cost-effective and delays diagnosis. 2, 3
- Serial repetitive testing: Avoid anxiety-provoking serial investigations once functional diagnosis is established. 1
- Relying on patient-reported food intolerances without objective testing: Leads to unnecessary dietary restrictions. 1
- Assuming normal CRP excludes IBD: Approximately 20% of active Crohn's disease patients have normal CRP. 2
- Fragmented specialist referrals: Patients with IBS often have multi-system complaints; avoid referring to different specialists for each new symptom. 1