Effectiveness and Safety of Acemetacin in Active Axial Spondyloarthritis: A Real-world Study

The goal of this observational study is to evaluate the effectiveness and safety of acemetacin in adults (18-65 years) with active axial spondyloarthritis (axSpA) who meet the 2025 ASAS-SPARTAN revised classification criteria and have an ASDAS score greater than 2.1. The main questions this study aims to answer are: * Does acemetacin reduce overall pain assessed by visual analog scale (VAS) after 4 weeks of treatment? * What proportion of patients achieve clinical remission (ASDAS ≤1.3) or low disease activity (1.3\<ASDAS≤2.1) at week 4? * What medical problems (side effects) occur during acemetacin treatment, with particular attention to gastrointestinal and cardiovascular events? Participants will: * Undergo screening assessments including blood tests, imaging of the sacroiliac joints (MRI, CT, X-ray), and physical examination within 7 days before starting treatment * Take acemetacin 90 mg by mouth once daily for 4 weeks * Complete a phone follow-up at week 2 and an in-clinic visit at week 4 * Have pain scores, disease activity measures (ASDAS, BASDAI, BASFI, ASAS HI, BASMI), and laboratory tests (CRP, ESR) recorded at each visit * Be monitored for adverse events throughout the treatment period