Use of the Bath Ankylosing Spondylitis Disease Activity Index in Patients With Psoriatic Arthritis With and Without Axial Disease.

OBJECTIVE: To evaluate whether the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) is a responsive instrument in psoriatic arthritis (PsA) and whether it differentiates between axial and peripheral disease activity in PsA. METHODS: Individuals with PsA initiating therapy in a longitudinal cohort study based in the United States were included. Axial PsA (axPsA), most often also associated with peripheral disease, was defined as fulfillment of the Assessment of Spondyloarthritis international Society axial spondyloarthritis classification criteria or presence of axial disease imaging features. Baseline BASDAI, individual BASDAI items, patient global assessment, patient pain, and Routine Assessment of Patient Index Data 3, and score changes following therapy initiation were descriptively reported. Standardized response means (SRMs) were calculated as the mean change divided by the SD of the change. RESULTS: The mean (SD) baseline BASDAI score at the time of therapy initiation was 5.0 (2.2) among those with axPsA (n = 40) and 4.8 (2.0) among those with peripheral-only disease (n = 79). There was no significant difference in patient-reported outcome scores between the groups. The mean change for BASDAI was similar among axial vs peripheral disease (-0.75 vs -0.83). SRMs were similar across axial vs peripheral disease for BASDAI (-0.37 vs -0.44) and the individual BASDAI items. CONCLUSION: BASDAI has reasonable responsiveness in PsA but does not differentiate between axPsA and peripheral PsA. (ClinicalTrials.gov: NCT03378336).