EXTENDED REPORT Development of the ASQoL: a quality of life instrument specific to ankylosing spondylitis

Background: Although disease-specific health status measures are available for ankylosing spondyli-tis (AS), no instrument exists for assessing quality of life (QoL) in the condition. Objective: To produce an AS-specific QoL measure that would be relevant and acceptable to respond-ents, valid, and reliable. Methods: The ASQoL employs the needs-based model of QoL and was developed in parallel in the UK and the Netherlands (NL). Content was derived from interviews with patients in each country. Face and content validity were assessed through patient field test interviews (UK and NL). A postal survey in the UK produced a more efficient version of the ASQoL, which was tested for scaling properties, reliability, internal consistency, and validity in a further postal survey in each country. Results: A 41 item questionnaire was derived from interview transcripts. Field testing interviews con-firmed acceptability. Rasch analysis of data from the first survey (n=121) produced a 26 item question-naire. Rasch analysis of data from the second survey (UK: n=164; NL: n=154) showed some item misfit, but showed that items formed a hierarchical order and were stable over time. Problematic items were removed giving an 18 item scale. Both language versions had excellent internal consistency (α=0.89–0.91), test-retest reliability (rs=0.92 UK and rs=0.91 NL), and validity