Objective: To investigate whether there are differential effects of disease activity and damage on physical functioning as measured by the Health Assessment Questionnaire (HAQ) over the course of psoriatic arthritis (PsA).
Methods: Between June 1993 and March 2005, 382 patients attending the University of Toronto PsA clinic had completed > or =2 HAQs on an annual basis. At the time of each HAQ assessment, clinical and laboratory measures of disease activity and damage were recorded. Generalized linear mixed-effects models were used to investigate the longitudinal relationship between disease activity, damage, and the HAQ score. To avoid floor effects that would arise in a single mixed-effects model, we adopted a 2-part model.
Results: The number of actively inflamed joints (measure of disease activity) and the number of clinically deformed joints (measure of damage) were positively and significantly related to the HAQ score. Furthermore, interaction terms for illness duration with the number of actively inflamed joints were statistically significant, with or without inclusion of the erythrocyte sedimentation rate and morning stiffness in the model (P = 0.029 and P < 0.001, respectively). The positive effects of actively inflamed joints on the level of the HAQ score decreased over increasing duration of PsA. There was less evidence to suggest that the positive effect of joint damage on the HAQ score increased over time.
Conclusion: Our results support the view that the influence of disease activity on HAQ scores declines with increased disease duration. We could not demonstrate strong evidence that the effect of clinical damage increases over the course of illness.