The hypothesis of the study is that the presence of (subclinical) gut inflammation at baseline in patients with early active axial spondyloarthritis predisposes to a more severe disease defined as more need to use anti-tumor necrosis factor α therapy and a shorter time to relapse after stopping anti-tumor necrosis factor α therapy after obtaining sustained clinical remission. Overall, the investigators hypothesize that subclinical gut inflammation is an important predictor in therapy response and outcome. These data could provide better insights into the complex interactions between gut and joint inflammation and guide the physicians in the therapeutic approach.