You may see short codes in a clinic letter, such as “A2 L3 B1” for Crohn’s disease or “E2” for ulcerative colitis. They come from the Montreal classification, agreed by IBD specialists in 2005, which records what kind of Crohn’s disease or ulcerative colitis someone has. It describes where the disease is and how it behaves; it is not a score of how active it is today.
Crohn’s disease: A, L and B
Each letter has a number:
- Age at diagnosis (A): A1 is 16 or younger, A2 is 17 to 40, and A3 is over 40.
- Location (L): L1 is the end of the small bowel (ileum), L2 is the large bowel (colon), and L3 is both. L4 means the upper gut is affected too, and is added to the others (for example “L3 + L4”).
- Behaviour (B): B1 is inflammation without narrowing or fistulas, B2 is narrowing (stricturing), and B3 is fistulas or abscesses (penetrating). A small “p” is added when the area around the anus is affected, as in “B1p”.
Ulcerative colitis: E and S
- Extent (E): E1 is the rectum only (proctitis), E2 is the left side of the colon, up to the bend near the spleen (left-sided colitis), and E3 is beyond that bend, often the whole colon (extensive colitis).
- Severity (S): S0 is remission with no symptoms; S1 is mild, with four or fewer stools a day and no signs of illness in the body; S2 is moderate, with more than four stools a day; S3 is severe, with six or more bloody stools a day plus signs such as a fast pulse, a raised temperature or anaemia.
Your IBD team can tell you what your own codes mean and whether they have changed over time: location and behaviour in Crohn’s disease, and extent in ulcerative colitis, can change.