Background The Psoriatic Arthritis Disease Activity Score (PASDAS) is a newly developed composite disease activity measure that summarizes psoriatic arthritis (PsA) disease activity with a score ranging from 0–10. PASDAS captures articular and extra-articular manifestations of the disease and the impact of the disease on the patient via the following variables: swollen and tender joints, dactylitis, Leeds enthesitis index, C-reactive protein, Health Assessment Questionnaire, physician and patient global disease activity, and the physical component summary score (PCS) of the medical outcomes survey Short Form 36 (SF-36). A limitation of PASDAS is that the score depends on the patient completing the SF-36, which requires a significant amount of time to complete. A shorter 12-question subset of SF-36, the SF-12, with the same range of values, agrees well with the SF-36 in many patient populations.
Objectives The objective was to measure the agreement between PASDAS calculated using the standard scoring formula and mPASDAS calculated by replacing the SF-36-PCS with SF-12-PCS.
Methods 100 PsA patients attending the University of Toronto PsA clinic for follow-up visits were consecutively recruited in June and July 2015. All variables required to calculate PASDAS were collected and PASDAS was calculated for each patient. The 12 item responses for SF-12 were extracted from the SF-36 questionnaires. A mPASDAS score was subsequently calculated based on the PASDAS scoring formula where SF-36 –PCS is replaced by SF-12 - PCS. A Bland-Altman plot of the differences in scores calculated via PASDAS and mPASDAS measured agreement between the two sets of scores. The rate of missclassification when categorizing the patient's disease activity state as high, moderate or low using the mPASDAS instead of PASDAS was also examined.
Results The analysis [N=100, 53% male, mean (SD) age 57.3 (11.9) years, mean (SD) disease duration 16.9 (11.7) years] revealed that the mean (SD) PASDAS was 3.29 (1.39) and the mean (SD) mPASDAS was 3.24 (1.27). The Bland-Altman plot showed a mean difference (95%CI) between mPASDAS and PASDAS of -0.05 (-0.07, -0.03). The lower limit and upper limits of the difference was (-0.24 [95%CI -0.21, -0.28]) and 0.14 [0.10, 0.17]), respectively. Five (5%) patients were misclassified (4 to the lower disease activity category) when mPASDAS was used instead of PASDAS.
Conclusions mPASDAS and PASDAS shows strong agreement without clinically significant systematic bias in mPASDAS scores. In clinical settings, the mPASDAS may replace PASDAS in disease activity assessment given the strong agreement and low misscalssification rate along with significantly reduced questionnaire burden.
Disclosure of Interest None declared