Objectives To assess the construct validity of the Simplified Ankylosing Spondylitis Disease Activity Score (SADSAS) to define disease activity and to compare the internal and external responsiveness of SADSAS with the Ankylosing Spondylitis Disease Activity Score (ASDAS) and the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) in an observational cohort of patients with axial spondyloarthritis (SpA).

Methods Three-hundred and ninety-seven patients with axial SpA, with disease duration of 5.1 years and never treated with tumor necrosis factor (TNF) blockers, were included in the study. One-hundred and fifty-six patients were observed longitudinally for 6 months. Clinical and laboratory outcome assessments were performed at baseline and at week 24. BASDAI, ASDAS based on CRP (ASDAS-CRP) or ESR (ASDAS-ESR) and SASDAS were evaluated. Construct convergent validity was calculated by correlating scores of the SASDAS index with ASDAS ERS/CRP and BASDAI. Responsiveness was assessed after six months of treatment with sulfasalazine (SSZ) or TNF-blockers. Internal responsiveness was evaluated by using the effect size (ES) and standardized response mean (SRM). External responsiveness was investigated by receiver operating characteristic (ROC) analysis. Changing scores were compared by calculating paired t-test statistic.

Results A strong correlations (p<0.0001) were observed between both SASDAS and ASDAS-ESR (r=0.835) and ASDAS-CRP (r=0.805). Strong correlations (p<0.0001) were also found between SASDAS and BASDAI score (r=-0.886). The cross-classification showed a significant overall agreement between SASDAS and ASDAS-ESR (chi-square =723.5; p<0.0001) and between SASDAS and ASDAS-CRP (chi-square =650.9; p<0.0001). The most efficient composite measure in detecting changes was the ASDAS-CRP (ES 1.95 and SRM 0.97). The responsiveness of SASDAS was slightly higher with comparison to ASDAS-ESR (ES of 1.62 and 1.33 and SRM of 0.88 and 0.71, respectively). Lack of responsivity was observed by using BASDAI (ES=0.93 and SRM =0.52). The area under ROC curve of the SASDAS showed similar results to those provided by ASDAS CRP/ESR. The score changes of all combinations were highly correlated (p<0.0001).

Conclusions The new SASDAS is a highly effective measure in assessing disease activity and it showed comparable internal and external responsiveness with respect to the ASDAS ESR/CRP response criteria in patients with axial SpA. SASDAS is easy to calculate and appear suitable for clinical decision making, epidemiologic research and clinical trials. Further longitudinal studies are needed to validate these encouraging results.

Disclosure of Interest None declared

DOI 10.1136/annrheumdis-2014-eular.5694