Description and prediction of physical functional disability in psoriatic arthritis: a longitudinal analysis using a Markov model approach.

OBJECTIVE To describe the longitudinal course of physical functioning in patients with psoriatic arthritis. METHODS Between June 1993 and June 2003, 341 patients attending the University of Toronto Psoriatic Arthritis Clinic completed 2 or more Health Assessment Questionnaires (HAQs). At the time of administration of each HAQ, patients were assigned to 1 of 3 physical functional disability states, based on their HAQ score. A Markov model that allowed for transitions to and from these 3 disability states was used to characterize the longitudinal course of physical functioning, as well as to identify factors for both progression and regression of disability. RESULTS Despite patient variability in the course of physical functioning, the following 3 longitudinal patterns were observed: 1) a stable state of disability throughout the entire study period, with 28%, 12%, and 6% of patients experiencing no, moderate, or severe disability, respectively; 2) a steady improvement or deterioration in disability over time (this pattern was observed in 27% of patients); and 3) a fluctuating state of disability, occurring in 27% of the patients. Sex, age, disease duration, number of actively inflamed joints, and number of deformed joints predicted transitions between disability states. CONCLUSION Although 28% of patients appeared resistant to becoming disabled over the duration of this study, the remaining patients were observed either to experience enduring disability or to move between disability states.