Understanding the drivers of BASDAI and back pain scores in psoriatic arthritis

The Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) is widely used to assess axial disease activity in psoriatic arthritis (PsA). However, 5 of its 6 questions reflect general disease activity rather than axial-specific symptoms. We aimed to evaluate the performance of BASDAI and its back pain subscore in assessing axial disease in PsA.

Patients with BASDAI scores were identified from a longitudinal PsA cohort initiated in 1978. Axial disease was defined radiographically. Trends in BASDAI and back pain scores were compared between patients with and without axial involvement. Associations of total BASDAI and back pain subscore with axial disease were assessed using univariable and multivariable linear mixed models in the entire cohort and stratified subgroups.

Of 1059 patients, 449 (42.4%) had axial and 610 (57.6%) had peripheral disease only. The mean age was 44.4 years (SD 12.8), and 55.9% were male. No difference in the BASDAI and back pain trends was observed between the axial and peripheral disease groups. Axial involvement was not associated with total BASDAI scores (β = -0.14, 95% CI -0.05 to 0.33). However, it was associated with a small, yet significant increase in back pain subscore (0.30, 0.06-0.55). Both BASDAI and back pain scores were associated with active peripheral joints, enthesitis, dactylitis, age, Psoriasis Area and Severity Index, and inversely with male sex. These associations were consistent across axial and peripheral disease subgroups.

BASDAI and its back pain subscore are influenced by peripheral musculoskeletal and skin disease activity in PsA, limiting their utility for assessing axial activity.