What is the diagnostic approach for a patient with suspected Irritable Bowel Syndrome (IBS)?

Diagnosing Irritable Bowel Syndrome

IBS is diagnosed positively based on characteristic symptoms—abdominal pain related to defecation or temporally associated with altered bowel habit—combined with limited baseline investigations to exclude organic disease, not as a diagnosis of exclusion. 1

Cardinal Symptoms Required for Diagnosis

The diagnosis hinges on confirming two essential features and their relationship 1:

• Recurrent abdominal pain (can be upper or lower abdomen) that is either relieved by defecation, exacerbated by defecation, or occurs when bowel habits worsen 1
• Altered bowel habit assessed using the Bristol stool chart—abnormal stool frequency and/or consistency 1
• The critical link: The patient must recognize a temporal relationship between their pain and bowel habit changes 1

Bloating is highly suggestive of IBS when present (often with visible distension), though not required for diagnosis 1

Essential History Elements

Document these specific features during the initial consultation 1:

• Onset and duration of symptoms (symptoms typically present >6 months) 1
• Post-infectious onset, recent antibiotic use, or onset after acute/chronic stress or psychological trauma 1
• Extraintestinal symptoms: back pain, urological symptoms, gynecological symptoms, insomnia 1
• Comorbid functional disorders: fibromyalgia, tension headache, chronic fatigue 1
• Psychological comorbidities and history of abuse (may require sensitive cueing) 1
• Medications particularly opioids and drugs affecting gut motility 1
• Family history of gastrointestinal cancer, IBD, or coeliac disease 1

Baseline Investigations (Primary or First Secondary Care Visit)

Perform these limited tests to exclude organic disease 1, 2:

• Full blood count 1
• C-reactive protein or ESR 1, 2
• Coeliac serology 1, 2
• Faecal calprotectin if diarrhea present AND age <45 years 1
  • If ≥250 μg/g: proceed to colonoscopy (high suspicion for IBD) 1
  • If 100-249 μg/g: repeat off NSAIDs/PPIs; refer for colonoscopy if remains elevated 1
  • If <100 μg/g: supports functional diagnosis 2

Alarm Features Requiring Further Investigation

These features mandate colonoscopy or additional workup, though 80% of IBS patients report at least one alarm symptom 1:

• Age >50 years at symptom onset 1
• Unintentional weight loss (document objectively) 1, 2
• Rectal bleeding 1, 2
• Nocturnal diarrhea or pain 1, 2
• Family history of colorectal cancer or IBD 1, 2
• Anemia on blood tests 1, 2
• Short symptom duration (<6 months) 1

When to Proceed with Colonoscopy

Consider colonoscopy in these specific scenarios 1:

• Suspected IBS-D with features of microscopic colitis: female sex, age ≥50 years, autoimmune disease, nocturnal/severe watery diarrhea, symptom duration <12 months, weight loss, or use of NSAIDs/PPIs/SSRIs/statins 1
• Any alarm features present 1
• Atypical features: nocturnal symptoms or obstructive defecation patterns 1

Important caveat: Colonoscopy yield in typical IBS is extremely low and does not provide patient reassurance 1

Subtype Classification

Use the Bristol stool chart to classify predominant stool pattern on days when stools are abnormal 1, 2:

• IBS-C (constipation): hard stools >25% of time, loose stools <25% of time 1
• IBS-D (diarrhea): loose stools >25% of time, hard stools <25% of time 1, 2
• IBS-M (mixed): both hard and loose stools >25% of time 1

Additional Testing for IBS-D Mimics

In suspected IBS-D with atypical features, consider bile acid diarrhea testing (SeHCAT scan or serum 7α-hydroxy-4-cholesten-3-one), as 25-33% of suspected IBS-D patients have abnormal bile acid retention 1

Making the Positive Diagnosis

When baseline investigations are normal and typical symptoms present without alarm features, confidently diagnose IBS based on symptoms alone 1. The Rome IV criteria validation showed patients meeting criteria were 21 times more likely to have IBS-C and 11 times more likely to have IBS-M after limited workup 1.

Critical Communication Points

Explain to patients 1:

• IBS is a chronic disorder of gut-brain interaction with fluctuating symptoms 1
• Not associated with increased cancer risk or mortality 1
• Affects quality of life similarly to organic diseases like IBD 1
• No cure exists, but treatments improve quality of life and are typically long-term 1
• Symptoms triggered by stress, illness, drugs, and eating 1