The International Classification of Diseases (ICD) diagnosis and Office of Population Censuses and Surveys (OPCS) operation codes used for the case and exposure conditions in this study were as follows:

DiagnosesICD 7ICD 8ICD 9ICD 10
Case conditions
Ulcerative colitis572.2563.1556K51
Crohn's disease572.0563.0555.0–555.2, 555.9K50
OperationsOPCS 1OPCS 2OPCS 3OPCS 4
The control conditions used in this study were as follows, using the terminology of ICD9 and the OPCS Classification of Surgical Operations, Third Revision: sebaceous cyst; disorders of tooth development and eruption; diseases of hard tissues of teeth; deflected nasal septum; nasal polyps; varicose veins of lower extremities; haemorrhoids; acute respiratory infections; strabismus; cataract; inguinal hernia; diseases of nail; internal derangement of knee; bunion; contraceptive management; dilatation of cervix and curettage of uterus; head injuries; fracture of upper limb and lower limb; disloations, sprains, and strains; superficial injury and contusion; total hip replacement; total knee replacement.
Appendicectomy441441, 443, 444441–444H01, H02
Tonsillectomy260–264230–236230–236F34, F36