Development of the ASQoL: a quality of life instrument specific to ankylosing spondylitis
- L C Doward1,
- A Spoorenberg2,
- S A Cook1,
- D Whalley1,
- P S Helliwell3,
- L J Kay4,
- S P McKenna1,
- A Tennant3,
- D van der Heijde2,
- M A Chamberlain3
- 1Galen Research, Enterprise House, Manchester Science Park, Lloyd Street North, Manchester M15 6SE, UK
- 2University Hospital, Maastricht, The Netherlands
- 3Rheumatology and Rehabilitation Research Unit, University of Leeds, Clarendon Road, Leeds, LS2 9NZ, UK
- 4Department of Rheumatology, Freeman Hospital, Newcastle-upon-Tyne, Newcastle NE7 7DN, UK
- Correspondence to:
Dr L C Doward, Galen Research, Enterprise House, Manchester Science Park, Lloyd Street North, Manchester M15 6SE, UK;
- Accepted 8 May 2002
Background: Although disease-specific health status measures are available for ankylosing spondylitis (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 respondents, 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 confirmed acceptability. Rasch analysis of data from the first survey (n=121) produced a 26 item questionnaire. 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.
Conclusions: The ASQoL provides a valuable tool for assessing the impact of interventions for AS and for evaluating models of service delivery. It is well accepted by patients, taking about four minutes to complete, and has excellent scaling and psychometric properties.
- AS, ankylosing spondylitis
- BASFI, Bath Ankylosing Spondylitis Functional Index
- DFI, Dougados Functional Index
- DIF, differential item functioning
- INFIT, information-weighted fit statistic
- IQR, interquartile range
- LDQ, Leeds Disability Questionnaire
- MNSQ, mean square
- NHP, Nottingham Health Profile
- NL, Netherlands
- OUTFIT, outlier-sensitive fit statistic
- QoL, quality of life