The purpose of this prospective cohort study is to evaluate the influence of serum drug levels and development of anti-drug antibodies on clinical response to anti-TNF agents in ankylosing spondylitis(AS) treatment. Secondary aims are to assess the demographic, clinical and laboratory variables associated with the development of anti-TNF drug antibodies at baseline or disease course and to reveal the impact of anti-drug antibodies on long-term efficacy or safety in particular drug survival in AS patients treated in daily clinical practice.