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Background Over the last decade, significant progresses have been achieved in the development and validation of new tools for the evaluation of disease activity in axial spondyloarthritis (SpA). Despite they play a key role in the assessment of these patients, the calculation scores are relatively complex and difficult to be quickly assessed in the busy daily clinical practice. Objectives To test the construct validity of the Simplified Ankylosing Spondylitis Disease Activity Score (SADSAS) to define disease activity and compare its internal and external responsiveness with the Ankylosing Spondylitis Disease Activity Score (ASDAS) and the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) in patients with axial SpA. Methods The patient cohort comprised 397 consecutive axial SpA patients who had never been treated with tumor necrosis factor (TNF) blockers. Clinical and laboratory outcome assessments were performed at baseline, and at week 24. The following parameters were evaluated: BASDAI, ASDAS-CRP, ASDAS-ESR, and SASDAS. Construct convergent validity was evaluated by correlating SASDAS with ASDAS ERS/CRP, BASDAI, Bath Ankylosing Spondylitis Functional Index (BASFI) and EuroQol five-dimensional (EQ-5D) questionnaire. One hundred and fifty-six patients were observed longitudinally for 6 months. Responsiveness was assessed after six months of treatment with sulfasalazine (SSZ) or biologics. 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. Change scores were compared by calculating paired t-test statistic for the difference. Results In testing for convergent validity 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), SASDAS and BASFI scores (rho = 0.588) and SASDAS and EQ-5D scores (rho = -0.579). The cross-classification showed a significant overall agreement (defined as the percentage of observed exact agreements) for SASDAS vs ASDAS-ESR (weighted k = 0.704) and for SASDAS vs ASDAS-CRP (k = 0.661). The most efficient composite measure in detecting change was the ASDAS-CRP (ES 1.95 and SRM 0.97). The responsiveness of SASDAS was slightly higher to ASDAS-ESR with an ES of 1.62 and 1.33, and an SRM of 0.88 and 0.71, respectively. The BASDAI appear to be the less responsive (ES = 0.93 and SRM = 0.52). The area under ROC curve of the SASDAS gives 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, therefore, appear suitable for clinical decision making, epidemiologic research, and clinical trials. [ABSTRACT FROM AUTHOR]