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Measuring Disability in Ankylosing Spondylitis: Comparison of Bath Ankylosing Spondylitis Functional Index with Revised Leeds Disability Questionnaire
SOPHIE EYRES, ALAN TENNANT, LESLEY KAY, ROBIN WAXMAN, and PHILIP S. HELLIWELL
ABSTRACT.
Methods. The psychometric properties of 2 measures of disability, the BASFI and the revised Leeds Disability Questionnaire (RLDQ), were compared in a cohort of 208 outpatients with AS. Rasch analysis was used to examine the properties of each measure and to compare them on a common scale. Test-retest was assessed in a cohort of 149 subjects who completed each instrument twice over an interval of 2 weeks. Results. Both instruments gave an even spread of scores across the study group, but BASFI responses were positively skewed and RLDQ responses negatively skewed. There was a highly significant difference between perceived severity groups for both instruments (Kruskal-Wallis chi-squared: RLDQ, 75.1; BASFI, 80.4; both p < 0.0001). Both instruments gave acceptable test-retest scores (RLDQ ICC = 0.95, 95% CI 0.93-0.97; BASFI ICC = 0.94, 95% CI 0.92-0.96). Both instruments were found to be unidimensional according to the Rasch model, but the BASFI had more items displaying differential item functioning. Category disordering was apparent with the BASFI but not the RLDQ. However, both instruments displayed disordered item thresholds. Neither instrument can be used as an interval measure. Both measures had "towers" of thresholds whereby several thresholds were marking the same point on the underlying disability construct. This was particularly notable in the case of the BASFI. Conclusion. Both the BASFI and RLDQ provide a unidimensional measure of function in AS that is in accord with patient perception of disease severity. Neither instrument can be used as an interval measure. Changing the way that the instruments are scored, for example by collapsing categories, may improve their performance. (J Rheumatol 2002;29:979-86) Key Indexing Terms:
ANKYLOSING SPONDYLITIS
From the Rheumatology and Rehabilitation Research Unit, University of Leeds, Leeds; and Freeman Hospital, Newcastle-upon-Tyne, England. S. Eyres, BSc, Research Fellow; A. Tennant, PhD, Professor of Rehabilitation Studies; R. Waxman, MPH, Research Fellow; P.S. Helliwell, MD, PhD, Senior Lecturer in Rheumatology, Rheumatology and Rehabilitation Research Unit, University of Leeds; L. Kay, MD, Consultant Rheumatologist, Freeman Hospital. Address reprint requests to Dr. P. Helliwell, Rheumatology and Rehabilitation Research Unit, University of Leeds, 36 Clarendon Road, Leeds, UK LS2 SNZ. E-mail: p.helliwell@leeds.ac.uk Submitted December 28, 2000; revision accepted November 25, 2001. |