Construct validity and responsiveness of ASAS Health Index assessed in two longitudinal studies of tumour necrosis factor alpha inhibitor initiation and dose reduction in patients with axial spondyloarthritis

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Abstract
Background The Assessment of SpondyloArthritis international Society Health Index (ASAS HI) is a novel questionnaire of global functioning for patients with axial spondyloarthritis (SpA). Objective The objective was to assess the construct validity, discriminatory ability and responsiveness of ASAS HI in relation to patient-reported outcome measures (PROMs), MRI and radiography. Methods Data from two longitudinal studies with tumour necrosis factor inhibitor (TNFi) initiation (novel MRI And biomarkers in Golimumab-treated patients with axial spondyloarthritis (MANGO): n=45) respectively tapering (Dose adjustment of Biological treatment in patients with SpA (DOBIS): n=106) were used. Analyses included a wide panel of PROMs, MRI and radiography scores of the spine and sacroiliac joints (SIJs). Results In the MANGO study, 30 (68%) patients were clinical responders at week 16. In the DOBIS study, 105 (99%) patients flared after mean (SD; min–max) 31 (17; 2.7–81) weeks. After initiation of TNF inhibitor in MANGO, ASAS HI significantly decreased from baseline to week 4, 16 and 52. In DOBIS, ASAS significantly increased from baseline to the flare visit and significantly decreased from the flare visit to week 96. In multivariate regression models, ASAS HI was independently associated with Spondyloarthritis Research Consortium of Canada MRI SIJ Inflammation score, Canada-Denmark MRI Spine Inflammation score, EuroQol, Ankylosing Spondylitis Disease Activity Index, Bath Ankylosing Spondylitis Disease Activity Index and Patient Global. Patients stratified according to ASAS HI health status groups (good, moderate, poor) at baseline and change categories (absolute and percentage change) from baseline to week 16/flare showed good discriminatory ability for almost all outcome variables (p≤0.001). ASAS HI had a large responsiveness in MANGO (standardised response mean (SRM)=−1.3, effect size (ES)=−1.7) and moderate responsiveness in DOBIS (SRM=0.7, ES=0.6). Conclusion ASAS HI showed good construct validity, discriminatory ability and responsiveness. Trial registration number ClinicalTrials.gov: NCT02011386.

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