To compare definitions of high disease activity of the Ankylosing Spondylitis Disease Activity Score (ASDAS) and Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) in selecting patients for treatment with biologic disease-modifying antirheumatic drugs (bDMARDs).
Patients from Rheumatic Diseases Portuguese Register (Reuma.pt) with a clinical diagnosis of axial spondyloarthritis (axSpA) were included. Four subgroups (cross-tabulation between ASDAS (≥2.1) and BASDAI (≥4) definitions of high disease activity) were compared regarding baseline characteristics and response to bDMARDs at 3 and 6 months estimated in multivariable regression models.
Of the 594 patients included, the majority (82%) had both BASDAI≥4 and ASDAS ≥2.1. The frequency of ASDAS ≥2.1, if BASDAI<4 was much larger than the opposite (ie, ASDAS <2.1, if BASDAI≥4): 62% vs 0.8%. Compared to patients fulfilling both definitions, those with ASDAS ≥2.1 only were more likely to be male (77% vs 51%), human leucocyte antigen B27 positive (79% vs 65%) and have a higher C reactive protein (2.9 (SD 3.5) vs 2.1 (2.9)). Among bDMARD-treated patients (n=359), responses across subgroups were globally overlapping, except for the most 'stringent' outcomes. Patients captured only by ASDAS responded better compared to patients fulfilling both definitions (eg, ASDAS inactive disease at 3 months: 61% vs 25% and at 6 months: 42% vs 25%).
The ASDAS definition of high disease activity is more inclusive than the BASDAI definition in selecting patients with axSpA for bDMARD treatment. The additionally 'captured' patients respond better and have higher likelihood of predictors thereof. These results support using ASDAS≥2.1 as a criterion for treatment decisions.