Diagnosis of Irritable Bowel Syndrome (IBS)
IBS should be diagnosed using the Rome III criteria, which requires recurrent abdominal pain or discomfort at least 3 days per month in the past 3 months, associated with two or more of: improvement with defecation, onset associated with change in stool frequency, or onset associated with change in stool form. 1
Diagnostic Criteria Evolution
The diagnostic approach to IBS has evolved through several iterations of criteria:
Manning Criteria
- Abdominal pain relieved by defecation
- Looser stools with onset of pain
- More frequent stools with onset of pain
- Abdominal distension
- Passage of mucus in stools
- Sensation of incomplete evacuation 1
Rome II Criteria
12 weeks or more in the last 12 months of abdominal discomfort or pain with two of three features:
- Relieved by defecation
- Associated with change in stool frequency
- Associated with change in stool consistency 1
Rome III Criteria (Current Standard)
Recurrent abdominal pain or discomfort at least 3 days per month in the past 3 months, associated with two or more of:
- Improvement with defecation
- Onset associated with change in frequency of stool
- Onset associated with change in form of stool 1, 2
Note: Criteria must be fulfilled for the past 3 months with symptom onset at least 6 months before diagnosis.
Making the Diagnosis
Positive Diagnostic Approach
- Identify typical symptoms matching Rome criteria
- Perform normal physical examination
- Confirm absence of alarm features:
- Weight loss
- Rectal bleeding
- Nocturnal symptoms
- Anemia 1
Supportive Features
The diagnosis is more likely if:
- Patient is female
- Age <45 years
- History >2 years
- Previous frequent consultations for non-gastrointestinal symptoms 1
When to Consider Further Testing
Further investigation is warranted if:
- Symptoms are atypical
- History is short (<6 months)
- Patient is over 45 years old
- Alarm features are present 1
Recommended Tests in Selected Cases
- Celiac disease screening
- Sigmoidoscopy (if colonic symptoms present)
- Thyroid function tests
- Stool microscopy
- Lactose tolerance testing (if substantial milk consumption)
- Colonoscopy or barium enema (for patients >45 years or with family history of colon cancer) 1
IBS Subtypes
IBS is classified into subtypes based on predominant stool pattern:
- IBS with constipation (IBS-C): Hard stools >25% of the time, loose stools <25% of the time
- IBS with diarrhea (IBS-D): Loose stools >25% of the time, hard stools <25% of the time
- Mixed IBS (IBS-M): Both hard and loose stools >25% of the time 2, 3
Common Pitfalls to Avoid
Overinvestigation: Once a functional diagnosis is established, the incidence of new non-functional diagnoses is extremely low 1
Ignoring red flags: Age >50 at symptom onset, short history, weight loss, nocturnal symptoms, family history of colon cancer, anemia, or rectal bleeding require further investigation 2
Confusing with other functional disorders: Diarrhea or constipation without abdominal pain are not IBS but separate functional disorders 2
Treating only one aspect: For IBS-C, treating constipation without addressing pain is inadequate 2
Repetitive testing: Avoid anxiety-provoking serial testing; perform necessary tests at the first visit 1
Treatment Approach
Treatment should target the predominant symptoms:
For All IBS Patients
- Positive diagnosis with clear explanation
- Reassurance about benign prognosis
- Listening to patient concerns
- Lifestyle modifications (stress reduction, physical activity) 1, 4
For IBS-C
For IBS-D
- First-line: Anti-diarrheal drugs (loperamide)
- Second-line: 5-HT3 antagonists, eluxadoline, rifaximin 5, 6
For Abdominal Pain
- First-line: Antispasmodics
- Second-line: Low-dose tricyclic antidepressants (e.g., amitriptyline) 5
The prognosis depends on the length of history and presence of chronic life stressors, with longer history and ongoing life stresses associated with poorer outcomes 1.