Pharmacokinetics of IFX and TNF Concentrations in Serum, Stool, and Colonic Mucosa in Acute Severe Ulcerative Colitis

Inclusion Criteria: * Present to hospital with ASUC based on Truelove and Witts criteria,33 defined as the presence of more than 6 bloody stools per day along with any 1 of the following: tachycardia \> 90 beats per minute, fever \> 37.8 °C, hemoglobin \< 10.5 g/dL, and erythrocyte sedimentation rate (ESR) \> 30 mm/h (or CRP \> 30 mg/L \[high-sensitivity CRP \> 300 mg/L\]) is a suitable surrogate if ESR is not available1). * Have a partial MCS \> 7. * Have a Mayo Clinic ES ≥ 2 with disease extending 15 cm or more beyond the anal verge. * Require rescue inpatient IFX infusion as part of routine care. Note, the IFX treatment regimen is not defined by this protocol and any dosage regimen is acceptable for the purposes of this study, such as standard or accelerated induction regimens. * Be able to speak English and participate fully in all aspects of this clinical trial. * Provide written informed consent. Exclusion Criteria: * A known history of being positive for anti-IFX antibodies. * Have a serious active infection, active malignancy, or any other known condition contraindicated with infliximab therapy, according to current prescribing information. * Serious underlying disease other than ASUC, or other physical or psychosocial condition that, in the opinion of the investigator, may interfere with the subject's ability to participate fully in the study. * Prior enrollment in the current study.