Performance-based functional assessment in ankylosing spondylitis: Clinical utility and validation of the Back Performance Scale (BPS-TR)

Background Self-report questionnaires are commonly used to assess functional status in patients with ankylosing spondylitis (AS). However, these instruments may not fully reflect patients’ actual physical performance. The Back Performance Scale (BPS-TR) may provide a more objective, performance-based approach to evaluating functional limitations. This study aimed to investigate the psychometric properties of the BPS-TR in patients with AS. Methods A total of 107 patients with AS were included. Functional performance was assessed using five standardized tasks: sock, pick-up, roll-up, fingertip-to-floor, and lifting tests. Convergent validity was examined using BASFI, BASDAI, ASQoL, and BASMI. Test–retest reliability was evaluated in 40 patients after a one-week interval using the intraclass correlation coefficient [ICC 2,1 ]. Results The BPS-TR demonstrated excellent test–retest reliability [ICC 2,1 = 0.890] and acceptable internal consistency (Cronbach's α = 0.787). Measurement error was small (SEM = 0.53; MDC 95 = 1.47). Significant moderate correlations were found with BASFI (r = 0.654), ASQoL (r = 0.590), BASDAI (r = 0.615), and BASMI (r = 0.630) (all p < 0.001). Exploratory factor analysis supported a unidimensional structure explaining 62.4% of the total variance. Conclusions The BPS-TR is a valid, reliable, and practical performance-based scale for assessing functional limitations in patients with AS. Its use alongside self-report questionnaires may contribute to a more comprehensive evaluation of functional status in clinical practice.