Fecal Calprotetin

This multicenter prospective cohort aims to evaluate whether combined changes in fecal calprotectin (FC) and fecal immunochemical test (FIT) at \*\*Week 2 and Week 4\*\* after initiating biologic therapy (vedolizumab or infliximab) can predict clinical response at \*\*Week 14\*\* and mucosal healing at \*\*Week 52\*\* in moderate-to-severe ulcerative colitis (UC) patients. Primary outcome: clinical remission rate at Week 14.

This multicenter prospective cohort study aims to evaluate whether a \>50% decrease or normalization of both quantitative fecal immunochemical test (qFIT) and fecal calprotectin (FC) levels at 2 weeks after starting conventional therapy (mesalazine or corticosteroids) can predict clinical relapse or need for biologic/JAK inhibitor therapy escalation by 52 weeks in biologic-naive patients with active ulcerative colitis (UC). Secondary objectives include assessing predictive value at 4 weeks, building dynamic prediction models, conducting health economic evaluation (Number Needed to Test, NNT), and exploring baseline predictors of early biomarker response. Patients will be observed during standard care with stool samples collected at Weeks 0, 2, and 4. Biomarker results will be blinded to clinicians/patients until study completion.