Cross-walk of the Assessment of Spondyloarthritis International Society Health Index and Ankylosing Spondylitis Quality of Life Scores in Ankylosing Spondylitis and Non-radiographic Axial Spondyloarthritis Patients

INTRODUCTION: Axial spondyloarthritis (axSpA) is a chronic rheumatic disease affecting the spine and sacroiliac joints, encompassing both ankylosing spondylitis (AS) and non-radiographic axial spondyloarthritis (nr-axSpA) patients. Patient quality of life (QoL) is assessed using the Ankylosing Spondylitis Quality of Life (ASQoL) (disease-specific measure) and the Assessment of SpondyloArthritis International Society Health Index (ASAS HI) (disease-specific measure). Both ASQoL and ASAS HI have similar parameters and scoring ranges, however, their performance relative to each other is unknown. We conducted a cross-walk analysis of the ASAS HI to the ASQoL in AS and nr-axSpA patients. METHODS: A cross-sectional survey using the Adelphi axSpA Disease Specific Programme™, conducted with rheumatologists and their consulting AS and nr-axSpA patients in the United States, was undertaken between Jun and Aug 2018. Rheumatologists provided confirmed diagnoses of AS and nr-axSpA alongside patients’ demographic and clinical characteristics. Patients reported quality-of-life measures using the validated ASAS HI and ASQoL questionnaires. Model performance was assessed by comparing root-mean squared error (RMSE) from tenfold cross-validation to determine the best mapping from ASAS HI to ASQoL, and vice versa. RMSE was calculated overall, and for lower, middle and upper thirds of the predicted scale. RESULTS: Data from 283 AS and 274 nr-axSpA patients were analyzed. Predicting ASAS HI using ASQoL values, the best model was non-parametric local linear regression, with overall RMSE of 1.851. Predicting ASQoL using ASAS HI values, the best model also used non-parametric local-linear regression, with overall RMSE of 2.254. In predicting ASAS HI and ASQoL, models performed better in predicting lower values in the predicted scale (RMSE 1.597, 1.871, 2.871 across thirds for ASAS HI; and 1.719, 2.577, 3.140 for ASQoL). CONCLUSIONS: Results present a scoring algorithm for cross-walking the ASAS HI to the ASQoL and vice versa, with the approach enabling comparisons to be made across studies.