Table 1.

Definition of used parelsnoer information model items

Diagnosis of IBD
Crohn’s disease Ulcerative colitis IBD-unclassified IBD-Indeterminate	Diagnosis of IBD was defined by clinical symptoms of the disease and confirmed by endoscopy, radiology or histology. If differentiation between CD and UC was not possible, patient were classified as Inflammatory Bowel Disease Unclassified (IBD-U). In the case that a pathologist was not able to differentiate between CD or UC following colectomy, the patient was classified as Inflammatory Bowel Disease Indeterminate (IBD-I).
Family history of IBD
	Family history of IBD was registered up to 3rd degree relatives
Smoking status
	Smoking status at diagnosis was documented. Patients were classified as never smokers, current smokers or former smokers
Education level
	Patients were classified in two groups; low education (Lower general education; Lower vocational education; General secondary education; Vocational secondary education; Did not finish primary school) and high education (Pre-university secondary education; Vocational post-secondary education; University)
Disease behavior	Disease behavior is reported if it occurred at any point in the disease course up to baseline.
Fistulising disease	Fistulising disease is confirmed by physical examination, radiological or endoscopy assessment.
Stricturing disease	A stricture had to be symptomatic. An anal stenosis had to be symptomatic and confirmed by physical examination.
Penetrating disease	Penetrating disease is defined as an intestinal abscess or intestinal perforation due to disease activity.
Prescribed medication
Anti-tumor necrosis factor alpha	Prescribed medication included anti-tumor necrosis factor alpha (TNF-alpha) agents (infliximab, adalimumab or certolizumab). Data on medication use was available for the entire disease course; medication use was therefore defined as medication ‘ever used’
Immunomodulators	immunomodulators (mercaptopurine, azathioprine, thioguanine or methotrexate). Data on medication use was available for the entire disease course; medication use was therefore defined as medication ‘ever used’
IBD-related surgical interventions
	IBD-related surgical interventions included small bowel resection, ileocecal resection, colon resection, resection other, strictureplasty, ileostomy/colostomy, surgery for abscesses or fistula, stoma and pouch
Extra-intestinal manifestations
Skin manifestations	Skin manifestations unrelated to the use of IBD medication such as pyoderma gangrenosum, erythema nodosum, hidradenitis suppurativa, psoriasis or palmoplantar psoriasiform pustolosis, and metastatic CD.
Musculoskeletal manifestations	Arthritis was defined as red and swollen joints, dactylitis, reactive arthritis and gout. Arthropathy was defined as painful joints, without any swelling or redness, with an inflammatory pattern: pain at night or at rest (e.g., sacroiliitis, ankylosing spondylitis, enthesitis, and inflammatory back pain).
Ocular manifestations	Ocular manifestations included uveitis and episcleritis.
Complications
Osteopenia	Osteopenia was defined as a bone mineral density T-score lower than -1.
Thromboembolic event	A thromboembolic event was confirmed by additional tests (radiology, endoscopy or histology).

IBD: Inflammatory bowel disease; CD: Crohn’s disease; UC: Ulcerative colitis.