ObjectivePhysical function in patients with axial spondyloarthritis (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 interrater reliability has not yet been determined. To our knowledge, this study is the first to evaluate the validity, reliability, and feasibility of the ASPI in another patient population, including both r− and nonradiographic (nr−) axSpA patients.MethodsPatients with axSpA were recruited from rheumatology clinics in Santiago, Chile. Dutch instructions were translated to Spanish by 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 3 ASPI tests. Validity was assessed through a patient questionnaire (numeric rating scale 0–10: ≥ 6 sufficient). For reliability, intraclass correlation coefficients (ICC) were calculated (with 95% CI). Correlations between the ASPI and disease variables were tested with regression analyses.ResultsSixty-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 interrater (n = 62) and test-retest (n = 39) reliability (ICC) of the 3 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 in < 15 min (feasibility).ConclusionThis study demonstrated a high validity and feasibility in an entirely different population, with both r-axSpA and nr-axSpA. The interrater and test-retest reliability was excellent. The ASPI instructions are now available for Spanish-speaking patients.