Objectives To evaluate the serum dosage of the biomedicine (DBM) and the incidence of antidrug antibody (ADA) against antitumor necrosis factor (TNF) in spondyloarthritis, and to demonstrate the influence of these parameters on the clinical efficiency. Methods We conducted a cross-sectional multicentric study including patients with spondylarthritis (SpA) under antiTNF (infliximab [INF], etanercept [ETA] and adalimumab [ADL]) for at least 6 months. A dosage of the ADA and DBM were practiced by the immuno-enzymatic essay. Result Seventy one patients were recruited. Disease modifying antirheumatic drugs (DMARDs) were associated with anti-TNF in 30%. ADA was positive in 54% for INF, 33% for ADL and 0% for ETA with a significant difference(p<0.0001). Immunogenicity was correlated to a bad therapeutic response (Bath Ankylosing Spondylitis Disease Activity Index [BASDAI]≥4)(p=0.04). The DBM was inversely correlated with the rate of ADA for patients treated with INF(p<0.0001) and ADL(p<0.0001). The DBM was also inversely correlated with BASDAI of INF(p=0.03) and ADL (p=0.01). ADA was significantly associated with an anterior switch of anti TNF(p=0.04), the use of INF(p=0.002), presence of coxitis(p=0.01) and higher body mass index (BMI)(p=0.007). DMARDs associated with anti TNF were not a protective factor for positive ADA. In a multivariate study, only INF and BMI were independent factors of positive ADA. Conclusion The ADA formation lowered the DBM and favored the therapeutic failure.