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.

This is a prospective, observational cohort study conducted across multiple centers in China. Biologic-naive UC patients (Mayo Endoscopic Subscore ≥2) initiating conventional therapy (mesalazine or corticosteroids) will be enrolled. Stool samples for quantitative FIT (qFIT) and fecal calprotectin (FC) will be collected at Baseline (Week 0) , Week 2 (±3 days) , and Week 4 (±3 days) . Patients are classified at Week 2:

Group A (Rapid Responders): Both qFIT & FC decrease >50% from baseline OR normalize.

Group B (Slow/Non-Responders): Either qFIT or FC decrease ≤50%. The primary endpoint is the composite event rate (clinical relapse OR treatment escalation to biologics/JAK inhibitors) by Week 52 (±2 weeks) . Clinical relapse is defined as an increase ≥2 points in partial Mayo score (excluding endoscopy) with a rectal bleeding subscore ≥1 requiring therapy adjustment. Treatment escalation occurs per standardized criteria (steroid-refractoriness or dependence). Secondary endpoints include time-to-event, corticosteroid-free remission, mucosal healing at Week 52, predictive model performance (AUC, sensitivity, specificity), NNT calculation, and baseline predictors.

Inclusion Criteria:

• Age 18-75 years.
• Established diagnosis of Ulcerative Colitis (UC) confirmed by clinical, endoscopic, and histopathological criteria.
• Endoscopic disease activity (Mayo Endoscopic Subscore ≥ 2) confirmed by colonoscopy during screening.
• Biologic-naive (no prior exposure to any biologic agent [e.g., infliximab, adalimumab, vedolizumab, ustekinumab] or JAK inhibitor).
• No use of systemic corticosteroids (oral or intravenous) within 4 weeks prior to Baseline (Week 0) visit.
• If using oral or rectal mesalazine/5-ASA preparations, dose must have been stable for ≥2 weeks prior to Baseline (Week 0).

Willing and able to provide written informed consent.

Exclusion Criteria:

• Diagnosis or high suspicion of Crohn's disease, ischemic colitis, infectious colitis, radiation colitis, intestinal tuberculosis, or other types of colitis.

Presence of other conditions clearly causing intestinal bleeding (e.g., acute hemorrhoidal bleeding, colorectal cancer, large colorectal polyps >1cm, intestinal vascular malformations).

• Untreated systemic conditions that may cause intestinal bleeding (e.g., thrombocytopenia [PLT <50 x 10^9/L], severe coagulopathy).
• Regular use of antiplatelet agents (e.g., aspirin, clopidogrel) or anticoagulants (e.g., warfarin, rivaroxaban).
• Pregnancy or lactation.
• Any other condition deemed by the investigator to make the patient unsuitable for study participation.

Ulcerative Colitis patints come to clinic or ward of hospitals.