Psychometric validation of the Bulgarian Ankylosing Spondylitis Quality of Life (ASQoL) questionnaire: linking quality of life with disease activity in axial spondyloarthritis

Assessing quality of life in patients with axial spondyloarthritis (axSpA) is essential for capturing the full burden of the disease beyond clinical and imaging findings. Quality of life measures support clinicians in identifying unmet patient needs and informing treatment decisions. The aim of this study was to develop and validate the first Bulgarian version of the Ankylosing Spondylitis Quality of Life (ASQoL) questionnaire. The development of the Bulgarian version of the ASQoL followed a three-stage process: translation, field testing, and psychometric evaluation. Following translation and field testing of the ASQoL, the psychometric evaluation involved administering the ASQoL to a random sample of axSpA patients on two occasions, 10–20 days apart, to assess its reliability and validity. Reliability was examined using internal consistency (Cronbach’s alpha) and test–retest reliability (Spearman’s rank correlation). Convergent validity was evaluated using the Short Form Health Survey Version 2 (SF-36v2) and the Ankylosing Spondylitis Disease Activity Score with C-reactive protein (ASDAS-CRP). Known-groups validity was assessed based on self-perceived general health and self-perceived disease activity. The Bulgarian version of the ASQoL demonstrated high internal consistency, with Cronbach’s alpha coefficients of 0.884 at time 1 and 0.882 at time 2. Test-retest reliability was excellent (Spearman’s ρ = 0.98, p < 0.001), indicating strong temporal stability. Convergent validity was supported by moderate correlations between ASQoL scores and relevant SF-36 domains, with the strongest associations observed for Physical Functioning, General Health, and Role Limitations. ASQoL scores showed a strong positive correlation with ASDAS-CRP (Spearman’s ρ = 0.70, p < 0.001), indicating close alignment between patient-reported quality of life and clinical disease activity. Known-groups validity was demonstrated by the ASQoL’s ability to distinguish between patient subgroups based on self-perceived general health and self-perceived disease activity, with statistically significant differences in ASQoL scores across general health categories. The Bulgarian ASQoL demonstrated strong psychometric properties, confirming its reliability and validity in accurately capturing the impact of axSpA on patients’ quality of life. Clinical trial number: not applicable.