Objectives To evaluate the validity of the Ankylosing Spondylitis Disease Activity Score (ASDAS) in early spondyloarthritis (SpA) in comparison with conventional clinical measures of disease activity.

Methods Six hundred and seventy-six incident cases of early SpA from the Esperanza programme were included. Patients were categorised into high and low disease activity states based on patient and physician global assessment scores and on the physician's decision to start treatment with a disease-modifying antirheumatic drug or tumour necrosis factor blocker. The discriminant ability of ASDAS-C-reactive protein (CRP) and ASDAS-erythrocyte sedimentation rate (ESR) was tested using standardised mean differences between patients with high and low disease activity. Convergent validity was tested by Pearson correlation between ASDAS versions and other measures of disease activity.

Results ASDAS-ESR and ASDAS-CRP showed good correlation with BASDAI (r=0.79 and 0.74, respectively). Both indices correlated well with the patient global assessment (r=0.70 in both indices) and moderately with the physician global score (r=0.46 and 0.47, respectively). CRP and ESR showed poor correlation with patient- and physician-derived measures. ASDAS performed similarly across the global SpA sample, ankylosing spondylitis (AS), non-radiographic axial SpA and peripheral SpA.

Conclusions ASDAS performed as a valid activity score even being slightly better than the Bath Ankylosing Spondylitis Disease Activity Index in its ability to discriminate between high and low disease activity in early SpA. ASDAS performed similarly in AS, early forms of SpA, non-radiographic axial SpA and peripheral SpA.