How is Irritable Bowel Syndrome (IBS) distinguished from Inflammatory Bowel Disease (IBD)?

Distinguishing Irritable Bowel Syndrome (IBS) from Inflammatory Bowel Disease (IBD)

The key distinction between IBS and IBD is that IBS is a functional disorder with no detectable structural abnormalities, while IBD is characterized by objective evidence of inflammation that can be detected through biomarkers, imaging, and endoscopy. 1, 2

Diagnostic Approach

Clinical Assessment

• IBS is diagnosed based on symptom criteria (Rome criteria), which include recurrent abdominal pain associated with defecation and changes in stool frequency or form 1
• IBD diagnosis requires objective evidence of inflammation through laboratory testing, imaging, and endoscopy 1
• Alarm features that suggest IBD rather than IBS include weight loss, nocturnal symptoms, bleeding, high-volume diarrhea, fevers, and acute symptom onset 1
• Age >50 years, family history of colon cancer, anemia, and rectal bleeding are red flags that warrant further investigation for IBD rather than IBS 1, 3

Laboratory Testing

• Fecal calprotectin is a crucial biomarker that helps distinguish between IBS and IBD:
  • <100 μg/g suggests IBS is likely 1
  • 100-250 μg/g represents an intermediate range requiring clinical correlation 1

  • 250 μg/g strongly suggests IBD and warrants urgent gastroenterology referral 1

• C-reactive protein (CRP) is often elevated in IBD but normal in IBS, though up to 15% of IBD patients may fail to mount a CRP response 1
• Basic laboratory workup should include complete blood count, electrolytes, CRP, and celiac screening to help differentiate IBS from IBD 1

Endoscopic Assessment

• Endoscopy with biopsy is essential for diagnosing IBD, showing characteristic mucosal changes and inflammation 1
• IBS patients have normal endoscopic findings without evidence of inflammation 1, 2
• Histologic examination of biopsies can detect microscopic inflammation in IBD even when mucosa appears normal endoscopically 1

Clinical Overlap and Challenges

IBS-like Symptoms in IBD

• Up to 27% of IBD patients with both endoscopic and histologic healing may still experience increased stool frequency and IBS-like symptoms 1
• IBD patients in clinical remission with persistent symptoms should be evaluated for ongoing subclinical inflammation before attributing symptoms to functional overlay 1
• The Crohn's Disease Activity Index (CDAI) may be elevated in both IBS and IBD patients, highlighting the importance of objective markers of inflammation 1

Pathophysiologic Distinctions

• IBD involves chronic mucosal and/or transmural inflammation with detectable immune activation 2, 4
• IBS pathophysiology may involve altered gut-brain axis, visceral hypersensitivity, and subtle immune activation that doesn't reach the threshold of detectable inflammation seen in IBD 1, 5
• Despite some theories, IBS is not a prodromal form of IBD, and they are not part of the same disease spectrum despite some overlapping symptoms 2, 5

Practical Diagnostic Algorithm

1. Initial symptom assessment:

   • Evaluate for Rome criteria for IBS 1
   • Screen for alarm features suggesting IBD 1

2. Non-invasive testing:

   • Measure fecal calprotectin and CRP 1
   • If calprotectin <100 μg/g and no alarm features, IBS is likely 1
   • If calprotectin >250 μg/g or presence of alarm features, refer for endoscopic evaluation 1

3. Endoscopic evaluation (when indicated):

   • Normal findings support IBS diagnosis 1
   • Evidence of inflammation supports IBD diagnosis 1

4. Ongoing assessment:

   • In IBD patients with persistent symptoms despite apparent remission, reassess for subclinical inflammation before attributing to IBS overlap 1
   • Consider that symptoms may be due to non-inflammatory mechanisms in IBD such as small intestinal bacterial overgrowth, bile acid diarrhea, or altered gut motility 1

Common Pitfalls to Avoid

• Relying solely on symptoms for diagnosis without objective testing can lead to misdiagnosis, as symptoms alone cannot reliably distinguish IBS from IBD 1
• Failing to use fecal calprotectin as a screening tool before invasive testing may lead to unnecessary procedures in IBS patients or delayed diagnosis in IBD patients 1
• Assuming that all symptoms in IBD patients in remission are due to functional overlay without excluding ongoing subclinical inflammation 1
• Not recognizing that therapeutic decisions in IBD should never be based on clinical symptoms alone without objective evidence of inflammation 1