The Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) is widely used for assessing disease activity in patients with axial spondyloarthritis (axSpA), particularly in settings where markers of inflammation are unavailable. As no consensus on BASDAI cut-off values exists for disease activity states in axSpA, we aimed to develop and validate such cut-offs against external criteria.

Routine care patients with axSpA initiating a biologic disease-modifying antirheumatic drug in eight European registries were included. Receiver operating characteristic analyses against external criteria were performed to determine optimal BASDAI values for separating remission, low disease activity (LDA), high disease activity (HDA), and very high disease activity (VHDA). Follow-up data at 6 months were used to select BASDAI cut-off values between remission and LDA and between LDA and HDA, whereas baseline data were used to select the cut-off for VHDA. The level of agreement between disease activity states based on BASDAI and Axial Spondyloarthritis Disease Activity Score (ASDAS) cut-off values was assessed using the proportion of discordance and weighted kappa.

In this cohort of 4,633 patients, the optimal BASDAI cut-off values between remission, LDA, HDA and VHDA were estimated to be <1.3, <2.5, and >5.3. The proportions of discordance between BASDAI and ASDAS disease activity states were 27.6% (weighted κ = 0.48) in baseline data and 37.6% (weighted κ = 0.28) in 6-month data.

BASDAI cut-off values for separating remission, LDA, HDA and VHDA were estimated in >4,600 patients. These cut-off values can be used for assessing disease activity and monitoring patients with axSpA, particularly when laboratory markers are unavailable.