Optimising Infliximab Induction Therapy for Acute Severe Ulcerative Colitis

Inclusion Criteria: * Age \>18 years old * Diagnosis of Ulcerative Colitis * Acute Severe Colitis according to the Truelove and Witt's Criteria * Steroid refractory according to the Oxford Criteria Exclusion Criteria: * Participant unable to consent for themselves * Indication for immediate surgery (acute abdomen, perforation of the bowel, haemorrhage) * Crohn's disease * Participants with enteric infection confirmed on stool microscopy, culture or toxin * Haemodynamic instability (mean arterial pressure \<60) and not responsive to fluids * Participants with clinically significant Cytomegalovirus infection (positive inclusion bodies, immunohistochemistry and signs of viraemia such as fever and abnormal liver function tests) * Participants who are pregnant or currently breast-feeding * Participants with current malignancy, excluding basal cell carcinoma * Participants with flat low or high grade colonic dysplasia; sporadic adenomas permitted * Participants with serious co-morbidities including: Immunodeficiency; Myocardial infarction or acute stroke within the last 3 months; Moderate or severe heart failure (New York Heart Association class III or IV); Active or suspected tuberculosis; Renal failure; Hepatic failure; other severe infections * Participants with history of hypersensitivity to infliximab or infliximab biosimilar * Participants who have received other immunosuppressive agents including but not limited to: Anti-TNF therapies within 3 months of screening (Infliximab, Infliximab biosimilar, Golimumab, Etanercept, Certolizumab or Adalimumab); Anti-integrins (Vedolizumb, Etrolizumab) within 4 months of screening; Calcineurin inhibitors (Cyclosporine, Tacrolimus) within 4 weeks of screening; T or B cell depleters (Rituximab, Alemtuzumab) within 12 months of screening; other investigational agents (eg. Ustekinumab) within 6 months of screening