Diagnostic Criteria for Irritable Bowel Syndrome
IBS is diagnosed using the Rome III criteria: recurrent abdominal pain or discomfort at least 3 days per month in the past 3 months, associated with two or more of the following: improvement with defecation, onset associated with a change in stool frequency, or onset associated with a change in stool form (appearance), with criteria fulfilled for the past 3 months and symptom onset at least 6 months before diagnosis. 1
Core Diagnostic Requirements
The diagnosis requires chronic, recurring abdominal pain or discomfort associated with disturbed bowel habit in the absence of structural abnormalities, with symptoms present for at least six months to distinguish IBS from transient conditions like infections or progressive diseases such as bowel cancer 1, 2.
Rome III Criteria Components
The patient must have recurrent abdominal pain or discomfort at least 3 days per month in the past 3 months associated with two or more of the following 1:
- Improvement with defecation (suggesting colonic origin) 1
- Onset associated with a change in frequency of stool 1
- Onset associated with a change in form (appearance) of stool (suggesting link to intestinal transit changes) 1
Important note: "Discomfort" means an uncomfortable sensation not described as pain 1.
Updated Rome IV Criteria
More recent guidance from the American Gastroenterological Association recommends at least 1 day of abdominal pain per week (rather than 3 days per month) to improve diagnostic accuracy 3. However, the Rome Foundation suggests that relaxing the frequency to 3 days per month may improve diagnostic performance without losing specificity 3.
Manning Criteria (Historical Context)
While the Rome criteria are preferred, the Manning criteria remain clinically useful and include six features 1:
- Pain relieved by defecation 1
- More frequent stools at onset of pain 1
- Looser stools at onset of pain 1
- Visible abdominal distension 1
- Passage of mucus per rectum 1
- Sense of incomplete evacuation 1
Studies requiring three Manning criteria give prevalences around 10%, though Manning criteria yield more variable results than Rome criteria 1.
Supportive Symptoms (Not Required for Diagnosis)
Common symptoms that support but are not part of diagnostic criteria include 1:
- Bloating 1
- Abnormal stool form (hard and/or loose) 1
- Abnormal stool frequency (<3/week or >3/day) 1
- Straining at defecation 1
- Urgency 1
Clinical Approach to Diagnosis
When to Make the Diagnosis in Primary Care
A working diagnosis can be safely made in general practice based on typical symptoms, normal physical examination, and absence of alarm features (weight loss, rectal bleeding, nocturnal symptoms, or anemia), confirmed by observation over time 1, 2.
Supportive Clinical Features
The diagnosis is more likely if the patient is 1:
- Female 1
- Aged <45 years 1
- History >2 years 1
- Frequent past attendance with non-gastrointestinal symptoms 1
Mandatory Investigations to Exclude Organic Disease
Before confirming IBS diagnosis, obtain 2:
- Complete blood count 2
- C-reactive protein or ESR 2
- Celiac serology (antiendomysial antibodies or tissue transglutaminase) 1, 2
- Fecal calprotectin 2
When to Refer for Further Investigation
Refer for sigmoidoscopy or colonoscopy if 1, 2:
- Age >45-50 years at symptom onset 1, 2
- Symptoms are atypical 1
- Short history 1
- Alarm features present: documented weight loss, nocturnal symptoms, rectal bleeding, anemia, fever, or family history of colon cancer or inflammatory bowel disease 2
Additional Testing in Specific Circumstances
- Stool microscopy for ova and parasites: if travel history or endemic area exposure 2
- Lactose tolerance testing: only if patient consumes substantial amounts (>0.5 pint/280 ml) of milk per day, as lactose malabsorption occurs in 8-25% depending on racial composition 1
- Thyroid function: reveals abnormalities in 1-2% of cases 1
- Urinary screen for laxatives: if surreptitious laxative abuse suspected 1
Critical Pitfalls to Avoid
Do not diagnose IBS if the duration criterion is not met (symptoms must be present for at least 6 months with criteria fulfilled for the past 3 months) 2. If symptoms are present for less than 6 months, reassess in 3-6 months, and if symptoms persist without alarm features and investigations are normal, then IBS diagnosis becomes appropriate 2.
Do not confuse IBS with functional dyspepsia: In IBS, abdominal pain is generally located in the lower abdomen and related to defecation, whereas in functional dyspepsia, pain is localized to the epigastrium and not related to the need to defecate 3. Up to 50% of patients have overlap with both conditions 3.
Do not perform repetitive, anxiety-provoking serial testing: If initial investigations are performed, they are best done on the first visit to avoid creating patient anxiety 1.
Prognosis Considerations
Prognosis depends on length of history (those with longer history less likely to improve) and chronic ongoing life stress (virtually precludes recovery—no patients with ongoing life stresses recovered over 16 months compared with 41% without such stresses) 1.