Multicentric randomized prospective study. Duration of inclusion 30 months. Duration of follow-up 24 months. 104 patients with spondyloarthritis treated with infliximab or adalimumab will be included if their ADAb dosage is positive, and they will be randomized (1:1) in two groups : "early switch" where treatment will be change to another anti-TNF, or "therapeutic intensification" where interval between two injections will be shortened. Patients will be evaluated clinically (ASDAS) and biologically (ADAb) at 12 weeks then at 24 weeks. Principal outcome will be the variation of ASDAS between baseline and end of the study. Number of patients to be included has been determined statistically from a preliminary study (power >98% for ASDAS variation of 20% on week 24).