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Montreal Classification for Inflammatory Bowel Disease (IBD)
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.