Crohn's vs Ulcerative Colitis: How to Tell Them Apart
Both are chronic inflammatory bowel diseases (IBD) that present with diarrhea, abdominal pain, and extraintestinal manifestations. They differ most in WHERE the inflammation sits and HOW DEEP it goes — and those two facts drive the endoscopy findings, complications, and workup.
How to tell them apart
| Feature | Crohn's disease | Ulcerative colitis |
|---|---|---|
| Distribution | Anywhere from mouth to anus; terminal ileum most common; SKIP lesions (patchy) | Colon only; CONTINUOUS, extending proximally from the rectum |
| Rectum | Often spared | Almost always involved |
| Depth of inflammation | Transmural (full thickness) | Mucosa and submucosa only |
| Endoscopy | Cobblestoning, aphthous ulcers, skip areas | Continuous friable mucosa, pseudopolyps, 'lead-pipe' colon |
| Histology | Non-caseating granulomas (not always present) | Crypt abscesses; no granulomas |
| Complications | Fistulae, strictures, abscesses, perianal disease, B12/malabsorption | Toxic megacolon, higher colorectal cancer risk, more severe bleeding |
| Serology | ASCA positive | p-ANCA positive |
| Smoking | Worsens disease | Paradoxically protective; onset often follows quitting |
| Bleeding | Less prominent | Bloody diarrhea is characteristic |
The reasoning
Anchor on distribution + depth. Continuous colonic inflammation starting at the rectum, sparing the small bowel, with bloody diarrhea → ulcerative colitis. Patchy skip lesions anywhere from mouth to anus, terminal-ileum involvement, and transmural disease with fistulae, strictures, or perianal involvement → Crohn's. Non-caseating granulomas confirm Crohn's when present. The smoking history is a useful tiebreaker: smoking worsens Crohn's but protects against UC.
Key tests
- Ileocolonoscopy with biopsies — the definitive test for pattern (continuous vs skip) and histology (granulomas vs crypt abscesses)
- CT or MR enterography — small-bowel Crohn's, fistulae, and strictures
- Fecal calprotectin and CRP — inflammation/activity
- ASCA / p-ANCA — supportive, never diagnostic
- Stool studies incl. C. difficile — always exclude infectious colitis first
What they share
- Chronic, relapsing-remitting inflammatory bowel disease
- Diarrhea and crampy abdominal pain
- Extraintestinal manifestations: arthritis, uveitis/episcleritis, erythema nodosum, pyoderma gangrenosum (primary sclerosing cholangitis is more associated with UC)
- Raised fecal calprotectin and CRP during flares
- Managed along a similar ladder: 5-ASA, corticosteroids, immunomodulators, biologics
Pitfalls
- ~10% of colitis is 'indeterminate' early — the pattern can take time to declare itself
- Backwash ileitis in UC can mimic small-bowel Crohn's
- ASCA/p-ANCA are supportive, not diagnostic — don't hang the diagnosis on them
- Always exclude infectious colitis (including C. difficile) before committing to IBD
Practice this the way the exam tests it — on branching cases where your decisions shape the patient.