Validity and reliability of the Ankylosing Spondylitis Disease Activity Score with C-reactive protein (ASDAS-CRP) and Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) in patients with axial spondyloarthritis (axSpA) in Singapore.

INTRODUCTION: The Ankylosing Spondylitis Disease Activity Score with C-reactive protein (ASDAS-CRP) and Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) are commonly used instruments for measuring disease activity. However, few studies have assessed their psychometric properties in patients with axial spondyloarthritis (axSpA). We aimed to assess the validity and reliability of ASDAS-CRP and BASDAI in patients with axSpA in Singapore. METHODS: Cross-sectional data from 280 patients with axSpA from a dedicated axSpA clinic in a Singapore tertiary referral hospital from 2011 to 2019 were used. Internal consistency was assessed using Cronbach's alpha. Construct validity was assessed through 12 a priori hypotheses by correlation of overall ASDAS-CRP and BASDAI score with other patient-reported outcomes measures (PROMs). Structural validity was evaluated via confirmatory factor analysis using maximum-likelihood method, where Comparative Fit Index (CFI) >0.95, Tucker-Lewis Index (TLI) >0.95, Root Mean Square Error of Approximation (RMSEA) <0.06 and Standardized Root Mean Residuals (SRMR) <0.08 were indicative of good fit. RESULTS: Among 280 patients (78.2% Male; 92.5% Chinese), ASDAS-CRP showed poor internal consistency of 0.33, while BASDAI showed high internal consistency of 0.87. Convergent and divergent construct validity were demonstrated by fulfillment of 11 out of 12 a priori hypotheses when ASDAS-CRP and BASDAI were compared with other PROMs. Our proposed ASDAS-CRP and BASDAI model showed good fit for a 1-factor structure respectively (CFI = 0.993, TLI = 0.984, RMSEA = 0.036, SRMR = 0.026 for ASDAS-CRP; CFI = 0.993, TLI = 0.985, RMSEA = 0.057, SRMR = 0.022 for BASDAI), demonstrating structural validity. CONCLUSION: This study supports the use of both ASDAS-CRP and BASDAI in measuring disease activity in patients with axSpA in Singapore.