⁃ \- 1. Adult patients, age 18 years or older, with Crohn's disease (CD), ulcerative colitis (UC) or Inflammatory Bowel Disease Unclassified (IBDU), who are either starting Zymfentra at week 10 (IFX-dyyb) in the setting of standard-of-care initiation with intravenous Infliximab (IFX) originator or IFX biosimilars induction therapy at weeks 0,2,6 or switching from intravenous IFX originator or IFX biosimilars during maintenance therapy to Zymfentra (IFX-dyyb) 2. Anticipation that the patient will be followed by the participating center for the next 12 months.

⁃ 3\. Diagnosis of CD, UC or IBDU must be established based on standard clinical, radiographic, endoscopic, and histologic criteria as described below.

⁃ The following diagnostic criteria were developed by the NIDDK IBD Genetics Consortium and are provided as guidelines to complete documentation on individuals with CD, UC or IBDU:

⁃ A) Symptoms including one or more: diarrhea, rectal bleeding, abdominal pain, fever, complicated perianal disease, extraintestinal manifestations, weight loss or failure to thrive.

⁃ AND B) Symptoms on two or more occasions separated by at least 8 weeks or ongoing symptoms of at least 6 weeks duration. When there has been a single episode of colitis (in some instances less than 6 weeks duration) resulting in colectomy and resolution of disease symptoms, pathology on the colectomy specimen should be consistent with idiopathic IBD and microbiology studies should be negative.

⁃ AND

⁃ C) One or more of the following providing objective evidence of inflammation:

⁃ Endoscopic: Mucosal edema, erythema, loss of normal submucosal vasculature, friability, ulceration, stricture formation, pseudopolyps, mucosal edema, erythema. Where there are only minor changes (mucosal edema, erythema, loss of normal submucosal vasculature, friability) mucosal biopsies should have been done to confirm the presence of IBD.

⁃ Radiologic: Mucosal thickening and/or nodularity, ulceration, stricture, pseudopolyps, fistula formation, pseudosacculation. Minor changes alone (mucosal thickening and/or nodularity) should not be sufficient to make a diagnosis of IBD.

⁃ Histologic: Mucosal erosion or ulceration, architectural changes of crypts, Paneth cell metaplasia (in colon), transmural inflammatory infiltrate\*, fibrosis of muscularis propria\*, noncaseating granuloma\*.

⁃ \* CD

⁃ Individuals with IBD should be classified into one of three categories, based on most recent diagnosis:

⁃ Crohn's disease (CD):

• Evidence of small intestinal inflammation with endoscopically, radiologically or histologically demonstrated ulcerations, fistulation, mucosal fissuring, nodularity or cobblestoning, stricture formation or histologically demonstrated transmural inflammation with or without granuloma formation.

• Isolated esophageal, gastric or duodenal inflammation with the finding of noncaseating granuloma.

• Colonic inflammation which is patchy (normal segments separating areas of inflammation, as described above) or associated with one or more of the following features: complete rectal sparing, multiple (\>10) aphthoid ulcers, deep ulceration (into the muscularis propria), transmural inflammation, extensive fibrosis and wall thickening, fistulation, non-caseating granuloma. (N.B. See note below regarding patchiness of endoscopically observed inflammation in patients with partially treated ulcerative colitis.)

• The presence of complex suppurative perianal disease (i.e. more than a superficial fistula or uncomplicated superficial abscess).

• If there are fewer than 10 aphthoid ulcers in the cecum (and the rest of the colon appears normal) in a patient with small bowel disease then this should be called small bowel disease only. Similarly, if the colon is normal except for the presence of a fistula extending from inflamed small bowel, the patient should be said to have small bowel disease alone. If the cecum is involved with ulcers larger than aphthoid ulcers or ulcers that are deep or if the involvement has resulted in deformity of the cecum this would be considered to be colonic involvement.

⁃ Ulcerative Colitis (UC)

⁃ 1\) Superficial inflammation and/or ulceration (involving only the mucosa and submucosa) of the colon which is continuous from the rectum extending proximally without skip lesions or complete rectal sparing (N.B. Relative rectal sparing is allowed for patients receiving topical rectal therapy; patchiness of endoscopic inflammation may be observed in patients with partially treated ulcerative colitis).

⁃ 2\) In patients with proctitis or left-sided ulcerative colitis there may be an area of inflammation in the cecum, usually surrounding the appendiceal orifice.

⁃ 3\) No inflammation of the small intestine (backwash ileitis is allowed - non-stricturing superficial inflammation of the terminal ileal mucosa associated with severe pancolitis which resolves following medical or surgical treatment of the colitis).

⁃ 4\) No features of Crohn's disease listed above.

⁃ Inflammatory Bowel Disease Unclassified (IBDU):

• Confirmed IBD by A, B and C above.

• Physician unable to classify individual into either CD or UC based on above criteria and/or patient has features of both CD and UC with none of the feature's diagnostic of one or the other.