Objective: To construct reference centile charts for validated measures of disease activity, functional impairment, and metrology (the Bath Ankylosing Spondylitis Disease Activity Index [BASDAI], Bath Ankylosing Spondylitis Functional Index [BASFI], and Bath Ankylosing Spondylitis Metrology Index [BASMI]) in a population of patients with AS, and to explore the potential application of these charts in patient care and clinical research.
Methods: We collected BASDAI and BASFI data on 2,979 patients with AS from the Royal National Hospital for Rheumatic Diseases (RNHRD) database and BASMI data on 393 patients at RNHRD. Data on age, sex, and duration of AS were also obtained. Centile charts were constructed for the BASDAI, BASFI, and BASMI in both males and females, with duration of AS as the time-dependant variable. Where necessary, BASDAI, BASFI, and BASMI raw scores were transformed to ensure that the indices were normally distributed over the range of disease duration, and allowance was made for change in variability of the indices.
Results: Linear models were fitted to the mean and standard deviations of BASDAI, BASFI, and BASMI scores in males and females as a function of disease duration. A standard deviation score plot confirmed goodness of fit of the models, and fitted centiles were derived. Charts showing the 5th, 10th, 25th, 50th, 75th, 90th, and 95th centiles were constructed. The charts confirmed that AS remains active after 40 years' duration (mean BASDAI score 2.22 and 2.99 in males and females, respectively). The charts also demonstrated that females have greater disease activity and more functional impairment than males, despite better metrology.
Conclusion: The use of centile reference charts for disease activity, functional impairment, and metrology in AS will allow physicians and patients to interpret index scores relative to a reference population and will provide the opportunity to assess change over time and response to therapeutic interventions, to improve definition of disease status, and to enhance patients' involvement in their care.