PT - JOURNAL ARTICLE AU - Thijs Willem Swinnen AU - Milica Milosevic AU - Sabine Van Huffel AU - Wim Dankaerts AU - Rene Westhovens AU - Kurt de Vlam TI - Bath Ankylosing Spondylitis Functional Index Shows Promising Reliability and Validity in the Assessment of Activity Limitations in Axial Spondyloarthritis AID - 10.3899/jrheum.150439 DP - 2016 Jun 15 TA - The Journal of Rheumatology PG - jrheum.150439 4099 - http://www.jrheum.org/content/early/2016/06/09/jrheum.150439.short 4100 - http://www.jrheum.org/content/early/2016/06/09/jrheum.150439.full AB - Objective The Bath Ankylosing Spondylitis Functional Index (BASFI) is the most popular method to assess activity capacity in axial spondyloarthritis (axSpA), to our knowledge. It is endorsed by the Assessment of Spondyloarthritis international Society. But it may have recall bias or aberrant self-judgments in individual patients. Therefore, we aimed to (1) develop the instrumented BASFI (iBASFI) by adding a body-worn accelerometer with automated algorithms to performance-based measurements (PBM), (2) study the iBASFI’s core psychometric properties, and (3) reduce the number of iBASFI items. Methods Twenty-eight patients with axSpA wore a 2-axial accelerometer while completing 12 PBM derived from the BASFI. A chronometer and both manual and “automated algorithm-based” acceleration segmentation identified movement time. Test-retest trials and methods (algorithm vs manual segmentation/chronometer/BASFI) were compared with ICC, standard error of measurement [percentage of movement time (SEM%)], and Spearman ρ correlation coefficients. Linear regression identified the optimal set of reliable iBASFI PBM. Results Good to excellent test-retest reliability was found for 8/12 iBASFI items (ICC range 0.812–0.997, SEM range 0.4–30.4%), typically with repeated and fast movements. Automated algorithms excellently mimicked manual segmentation (ICC range 0.900–0.998) and the chronometer (ICC range 0.878–0.998) for 10/12 iBASFI items. Construct validity compared with the BASFI was confirmed for 7/12 iBASFI items (δ range 0.504–0.755). Together, sit-to-stand speed test (stBeta 0.483), cervical rotation (stBeta –0.392), and height (stBeta –0.375) explained 59% of the variance in the BASFI (p < 0.01). Conclusion The proof-of-concept iBASFI showed promising reliability and validity in measuring activity capacity. The number of the iBASFI’s PBM may be minimized, but further validation in larger axSpA cohorts is needed before its clinical use.