Ankylosing Spondylitis (AS) / Radiographic Axial SpA (r-axSpA)

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

The Chinese Spondyloarthritis Inception cohort (CESPIC) was started 2000 as a prospective, longitudinal, multicentre, nationwide study in China on patients with early SpA including ankylosing spondylitis (AS, also known as radiographic axial spondyloarthritis) and non-radiographic axial SpA. The objectives of CESPIC are to learn about the course of SpA during the very early stage of the disease, to appropriately assess the outcome including radiographic progression of patients after several years of follow-up, to identify outcome predictors, to assess quality of life, function, and costs (direct and indirect costs). CESPIC has been recently expanded to recruit patients with other forms of SpA / conditions associated with SpA: reactive arthritis, acute anterior uveitis, Crohn's disease as well as with psoriasis / axial psoriatic arthritis.

The German Spondyloarthritis Inception cohort (GESPIC) was started 2000 as a prospective, longitudinal, multicentre, nationwide study in Germany on patients with early SpA including ankylosing spondylitis (AS, also known as radiographic axial spondyloarthritis) and non-radiographic axial SpA. The objectives of GESPIC are to learn about the course of SpA during the very early stage of the disease, to appropriately assess the outcome including radiographic progression of patients after several years of follow-up, to identify outcome predictors, to assess quality of life, function, and costs (direct and indirect costs). GESPIC has been recently expanded to recruit patients with other forms of SpA / conditions associated with SpA: acute anterior uveitis, Crohn's disease as well as with psoriasis / axial psoriatic arthritis.