Two Conditions, Confusing Overlap
Irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD) share superficially similar symptoms — abdominal pain, bloating, altered bowel habits — yet they represent fundamentally different disease categories. IBS is a disorder of gut–brain interaction characterised by altered motility and visceral hypersensitivity without macroscopic tissue damage. IBD (encompassing Crohn's disease and ulcerative colitis) is a chronic immune-mediated condition producing visible mucosal inflammation, ulceration, and, over time, structural complications such as strictures, fistulae, and fibrosis.
Pathology: Functional vs Inflammatory
The core distinction lies in what happens at the tissue level. In IBD, colonoscopy and histology reveal crypt abscesses, granulomata (in Crohn's), and transmural or mucosal inflammatory infiltrates. Biomarkers — fecal calprotectin, C-reactive protein, erythrocyte sedimentation rate — are characteristically elevated during active disease. In IBS, these investigations are normal or near-normal, because the pathology operates at the level of neural signalling and motility rather than immune-mediated tissue destruction.
Diagnostic Approach
Current guidelines recommend a positive diagnostic strategy for IBS using Rome IV criteria when alarm features are absent. For suspected IBD, the combination of raised inflammatory markers (particularly fecal calprotectin > 200 µg/g) and clinical features prompts endoscopic evaluation with biopsies. The differential becomes challenging when IBS-like symptoms persist in IBD patients who are in endoscopic remission — a phenomenon affecting up to 40% of IBD patients.
The Overlap Problem
IBS and IBD are not mutually exclusive. Studies using the Rome criteria estimate that IBS-type symptoms occur in 30–40% of IBD patients even during remission. This so-called "IBS-IBD overlap" may reflect residual visceral hypersensitivity, post-inflammatory neural remodelling, bile-acid malabsorption, small intestinal bacterial overgrowth, or persistent subclinical inflammation not captured by standard endoscopy. Distinguishing true IBD relapse from superimposed IBS-type symptoms is critical, because the treatment pathways diverge: immunosuppressive escalation for the former, neuromodulation and dietary management for the latter.
Decision-Making Framework
A structured clinical approach starts with biomarkers. If fecal calprotectin is normal and there are no alarm features, IBS management should be prioritised. If calprotectin is elevated, endoscopic reassessment is warranted. When symptoms persist despite quiescent endoscopic disease, clinicians should consider IBS-type management layered on top of ongoing IBD therapy — a nuanced, patient-centred strategy rather than a binary choice.