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Montreal Classification for Inflammatory Bowel Disease (IBD)
Phenotypic classification of Crohn's disease and ulcerative colitis by age, location, behaviour, extent and severity.
Montreal Classification for Inflammatory Bowel Disease (IBD)
Disease type
Age at diagnosis (A) — Crohn's disease
Disease location (L) — Crohn's disease
Concomitant upper GI disease modifier (L4)
L4 is added to L1–L3 whenever disease also involves the upper GI tract proximal to the terminal ileum (it is a modifier, not an isolated category).
Disease behaviour (B) — Crohn's disease
Perianal disease modifier (p)
Disease extent (E) — Ulcerative colitis
Disease severity (S) — Ulcerative colitis
ResultA1L1B1
Crohn's disease classified by age at diagnosis (A), location (L, ± L4 upper GI modifier) and behaviour (B, ± perianal modifier p).
- Age at diagnosis
- A1
- Location
- L1
- Behaviour
- B1
When to use
- Standardising phenotypic description of Crohn's disease or ulcerative colitis for communication, research and trial enrolment.
Formula
Crohn's disease: A (age at diagnosis) + L (location, ±L4 modifier) + B (behaviour, ±p perianal modifier). Ulcerative colitis: E (extent) + S (severity).
Pearls and pitfalls
- L4 and p are modifiers added to the location/behaviour code rather than mutually exclusive categories.
- Behaviour can change over time (e.g. B1 progressing to B2/B3), so re-classification during follow-up is expected.