In case of acute severe colitis on day 3 of hospital , the best predictor of colectomy is:

C-reactive protein is a less good marker for assessing disease activity in UC than Crohn's disease. In acute severe colitis a raised CRP > 45 mg/L on day 3 following hospital admission together with 3-8 stools a day is highly predictive for colectomy. Ref: Journal of Crohn's and Colitis (2012) 6, 965-990.