Objectives To investigate the relationship between MRI inflammation and measures of clinical disease activity as well as treatment responses in patients with ankylosing spondylitis (AS) treated with a tumour necrosis factor inhibitor.

Methods MRI at baseline (n=221), 24 (n=158) and 102 weeks (n=179) were scored for inflammation/activity (MRIa, Berlin scoring system). Treatment responses according to the AS disease activity score (ASDAS), Bath AS disease activity index (BASDAI) and assessment of spondyloarthritis 20 (ASAS20) criteria were calculated. For each treatment response criterion, subgroups of responders and non-responders changes in MRIa scores were compared.

Results Higher baseline ASDAS and C-reactive protein (CRP) values were associated with higher baseline MRIa scores and with greater decreases in MRIa scores at follow-up. ASDAS and CRP improvements correlated with MRIa improvement. Stronger correlations were observed for CRP. Differences in MRIa change scores between responders and non-responders were greater when subgroups were defined according to ASDAS response than according to BASDAI or ASAS20 response.

Conclusions MRIa correlates better with CRP than with other measures of disease activity. By including both CRP and patient-reported outcomes in its formula, ASDAS has the advantage of providing combined information on objective and subjective measures. As a status and response measure ASDAS better reflects the spinal inflammatory disease process in AS than other composite measures.