The objective 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. The assessment of demographic, clinical and laboratory variables associated with the development of anti-TNF drug antibodies at baseline or disease course will be also evaluated. Via this study, it might also be possible 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.