Ankylosing Spondylitis Physical Performance Index (ASPI): objective test for physical functioning is highly reliable and feasible.

Objective Physical function in axial spondyloarthritis patients (axSpA) is currently evaluated through questionnaires. The Ankylosing Spondylitis Performance Index (ASPI) is a performance-based measure for physical functioning, which has been validated in Dutch patients with radiographic (r-)axSpA. The inter-rater reliability was not yet determined. This study is the first to evaluate the validity, reliability and feasibility of the ASPI in another patient population, including both radiographic- and non-radiographic axSpA patients. Methods AxSpA patients were recruited from Rheumatology clinics in Santiago, Chile. Dutch instructions were translated to Spanish via a forward-backward procedure. Study visits were performed at baseline and 1-4 weeks later. Four ASPI observers were involved, measuring the performance times of the three ASPI tests. Validity was assessed through a patient questionnaire (NRS 0-10: ≥6 ‘sufficient’). For reliability, intra class correlation (ICC) coefficients were calculated (with 95% CI). Correlations between the ASPI and disease parameters were tested with regression analyses. Results Sixty-eight patients were included (57% male, 52% r-axSpA). All patients understood the Spanish instructions and considered the ASPI to reach its aim (84%) and representativeness (85%) for physical functioning. The overall inter-rater- (n=62) and test-retest (n=39) reliability (ICC) of the three tests combined were 0.93 (0.88-0.96) and 0.94 (0.87-0.97), respectively. Eighty-two percent of the patients completed all tests and 94% finished Conclusion This study demonstrated a high validity and feasibility in an entirely different population, with both r- and nr-axSpA. The inter-rater- and test-retest reliability was excellent. The ASPI instructions are now available for Spanish-speaking patients.