Simplified Ankylosing Spondylitis Disease Activity Score (SASDAS) Versus ASDAS: A Post Hoc Analysis of a Randomized Controlled Trial

Objective To compare the Simplified Ankylosing Spondylitis Disease Activity Score (SASDAS) against the Ankylosing Spondylitis Disease Activity Score (ASDAS) for measuring and categorizing disease activity using data from the EMBARK trial ( ClinicalTrials.gov : NCT01258738 ), a randomized controlled trial of etanercept (ETN) for axial spondyloarthritis (axSpA). Methods Patients with early active axSpA received ETN 50 mg once weekly (n = 106) or placebo (PBO; n = 109) for 12 weeks in a double-blind manner; they then received open-label ETN for 92 weeks. For this analysis, ASDAS–C-reactive protein (CRP) and SASDAS-CRP were calculated at baseline, week 12, and week 24. The SASDAS was calculated by the linear addition of the ASDAS components without adjustment. Results A very strong correlation, as determined by the Spearman correlation coefficient, was observed between the ASDAS and SASDAS for continuous variables at baseline and during treatment. For pooled categorical data at baseline, the SASDAS placed 69.9% of patients in the same disease categories as the ASDAS but overestimated for 17.8% of patients and underestimated for 12.2% of patients. A similar pattern was seen postbaseline. Cohen weighted statistics for all individual and pooled treatments and timepoints (0.54-0.73) reflected moderate to substantial agreement. The capacity to differentiate between treatments (ie, ETN and PBO/ETN) was higher with the ASDAS (effect size −0.74, 95% CI −1.03 to −0.46) compared with the SASDAS (effect size −0.51, 95% CI −0.79 to −0.23), but sensitivity to change was generally similar. Conclusion A very strong correlation between the SASDAS and ASDAS was observed when considering continuous variables; however, moderate to substantial agreement was observed for categorical data, and the SASDAS classified a lower proportion of patients as being in the inactive and low disease activity categories.