EXTENDED REPORT Assessment of disability with the World Health Organisation Disability Assessment Schedule II in patients with ankylosing spondylitis

Objective: To investigate in ankylosing spondylitis (AS) whether the newly developed World Health Organisation Disability Assessment Schedule II (WHODAS II) is a useful instrument for measuring disability, to assess its responsiveness in relation to other traditional disease specific instruments, and to identify factors that are associated with both short term and long term scores on the WHODAS II. Methods: Patients with AS from a randomised controlled trial assessing the efficacy of spa treatment (n=117) and from a five year longitudinal observational study (n=97) participated. The patients completed several questionnaires, including the WHODAS II. After a three week course of spa treatment, 31 patients again completed all questionnaires to assess responsiveness. To determine to what degree the WHODAS II reflects some AS oriented measures on disease activity, functioning, and quality of life, correlation coefficients between the WHODAS II and these other questionnaires were calculated. Responsiveness was calculated by the effect size (ES) and standardised response mean (SRM). Linear regression analysis was performed to explore which factors might be associated with short term changes on the WHODAS II and to investigate (in the observational study) which factors of WHODAS II might predict disability five years later. Results: Mean score on the WHODAS II was 23.9 (SD 15.5 (range 0.0−76.1)). Scores on the WHODAS II were significantly correlated with all disease specific questionnaires measured (all p<0.001). The WHODAS II showed a comparable short term responsiveness score (SRM 0.41; ES 0.39). In regression analysis these short term changes on the WHODAS II were significantly associated with changes in functioning (β coefficient 4.25, 95% confidence interval (95% CI) 1.24 to 7.26, p=0.007). In the observational study, disease activity (β coefficient 0.35, 95% CI 0.17 to 0.53, p<0.000) as well as functioning (β coefficient 0.23, 95% CI 0.09 to 0.38, p=0.002) seemed to significantly predict disability (WHODAS II) after five years. Conclusion: The WHODAS II is a useful instrument for measuring disability in AS in that it accurately reflects disease specific instruments and that it shows similar responsiveness scores. In AS, a short term change on the WHODAS II is associated with a change in physical function. At the group level, disease activity and physical functioning may predict disability after five years.