What are the diagnostic criteria and treatment options for Irritable Bowel Syndrome (IBS)?

How to Diagnose IBS

IBS can be diagnosed clinically in primary care based on the presence of recurrent abdominal pain at least 1 day per week in the last 3 months (with symptom onset at least 6 months prior), associated with two or more of the following: pain related to defecation, change in stool frequency, or change in stool form, combined with normal baseline investigations and absence of red flag features. 1, 2

Cardinal Symptoms Required for Diagnosis

The diagnosis hinges on confirming the relationship between two core features 1:

• Abdominal pain (upper or lower abdomen) that is relieved or exacerbated by defecation, or associates temporally with changes in bowel habit 1
• Altered bowel habit with abnormal stool frequency and/or consistency using the Bristol stool chart 1

The patient must recognize the link between their pain and bowel habit changes—for example, pain worsens when more constipated or when diarrhea worsens 1.

Initial Clinical Assessment

History Taking Essentials

Focus on these specific elements 1:

• Onset and duration: When symptoms started and how long they've persisted 1
• Temporal relationship: Confirm the patient recognizes pain correlates with bowel habit changes 1
• Predominant stool pattern: Classify using Bristol stool chart on days when stools are abnormal 1
• Bloating: While not required for diagnosis, its presence is highly suggestive of IBS and often accompanied by visible distension 1
• Triggering events: Post-infectious onset, recent antibiotic use, acute/chronic stress, or psychological trauma 1
• Extraintestinal symptoms: Back pain, urological symptoms, gynecological symptoms, insomnia 1
• Medication review: Specifically ask about opioids 1

Red Flags That Require Further Investigation

The diagnosis can be made safely only in the absence of these sinister features 1:

• Weight loss (unintentional and documented) 1
• Rectal bleeding or hematochezia 1, 3
• Nocturnal symptoms 1
• Anemia 1
• Family history of colorectal cancer or inflammatory bowel disease 1, 3

Supportive Clinical Features

The diagnosis is more likely if 1:

• Female sex 1, 2
• Age <45 years with symptom duration >2 years 1, 2
• History of frequent healthcare visits for non-gastrointestinal symptoms 1, 2

Baseline Investigations Required

Perform these tests at the first visit to avoid repetitive, anxiety-provoking serial testing 1:

• Full blood count 1
• C-reactive protein or erythrocyte sedimentation rate 1
• Coeliac serology (tissue transglutaminase antibodies with total IgA level) 1
• Faecal calprotectin if diarrhea present and age <45 years 1

Interpreting Faecal Calprotectin Results

1:

• If ≥250 μg/g: High suspicion for IBD, proceed to colonoscopy
• If 100-249 μg/g (indeterminate): Repeat test off NSAIDs and proton pump inhibitors; refer for colonoscopy if repeat remains indeterminate or abnormal
• If normal: Supports IBS diagnosis

When to Refer for Further Investigation

Refer for hospital evaluation and additional testing if 1:

• Atypical symptoms 1
• Short symptom duration 1
• Age >45 years at symptom onset 1

Additional Investigations in Secondary Care

When referred, patients typically require 1:

• Sigmoidoscopy if colonic symptoms present; biopsy any abnormality and all patients with diarrhea to detect microscopic colitis 1
• Colonoscopy or barium enema if family history of colon cancer or age >45 years at onset with colonic symptoms 1
• Thyroid function tests (reveals abnormalities in 1-2% of cases) 1
• Stool microscopy for parasites 1

Consider bile acid malabsorption testing (SeHCAT scan) in patients with IBS-D, as one-in-three to one-in-four have abnormal retention 1.

Making the Positive Diagnosis

Once baseline investigations are normal and red flags excluded 1:

1. Make a positive diagnosis rather than a diagnosis of exclusion 1
2. Explain the diagnosis in language the patient understands 1
3. Provide reassurance of benign prognosis 1
4. Listen to patient concerns and address their fears and beliefs 1
5. Confirm diagnosis over time through observation in primary care 1

Common Pitfalls to Avoid

• Over-investigation: Once a functional diagnosis is established, the incidence of new non-functional diagnoses is extremely low 1, 2
• Rigid adherence to criteria: The Rome criteria were initially developed for research; many patients with similar clinical course don't fit exactly but still have IBS 1
• Missing the pain-bowel habit relationship: Without abdominal pain, IBS diagnosis is untenable 3
• Ignoring symptom fluctuation: Symptoms and subtypes often change over time 2
• Overlooking psychological comorbidity: Common mental disorders and history of abuse frequently coexist 1

Subtyping After Diagnosis

Classify based on predominant stool pattern on abnormal days 2:

• IBS with constipation (IBS-C)
• IBS with diarrhea (IBS-D)
• Mixed IBS (IBS-M)
• Unsubtyped IBS

This classification guides treatment selection but may change over time 2.