Objective To describe the prevalence of fibromyalgia (FM) in an axial spondyloarthritis (axSpA) population and to confirm that concomitant FM had a negative impact on tumour necrosis factor blockers’ (TNFb) response.

Design Prospective observational study with two visits 3 months apart.

Patients Adult patients with AxSpa initiating a TNFb.

Study groups FM was defined by the Fibromyalgia Rapid Screening Tool (FiRST) at baseline and also by a sustained positive FiRST (both visits) and by a fulfilment of the 1990 American College of Rheumatology criteria for FM.

Statistical analysis Prevalence of FM; evaluation of the impact of a concomitant FM on TNFb response (Bath Ankylosing Spondylitis Disease Activity Index (BASDAI 50) as primary endpoint), adjusted by factors known to have an impact on TNFb response.

Results Among the 508 patients included in the main analysis, 192 (37.8%) were screened at baseline as FM. Percentage of success after 12 weeks of treatment was lower in the FM group for most of the effectiveness endpoints (eg, BASDAI 50: 45.3% vs 54.1% in the FM/not FM groups according to the FiRST), except for the C reactive protein change endpoints which were not different across groups.

Conclusion This study confirms that FM coexists in patients with axSpA and that its presence seems to have a negative impact on TNFb response, which seems more related to the self-reported instruments used in its evaluation, rather than a different treatment effect of the molecule in this subgroup of patients.