Objective: To define the minimum clinically important improvement (MCII) and minimum clinically important worsening (MCIW) for the Patient Activity Scale II (PAS-II; range 0–10), a recommended patient-reported outcome measuring rheumatoid arthritis disease activity. Methods: Data were taken from Forward, The National Databank for Rheumatic Diseases, from four 6-month data collection periods. Both anchor-based and distribution-based methods were used to estimate the MCII and MCIW. Anchor-based analyses used comparisons of pain and general health to the previous 6 months. Distribution-based analyses used 0.5 and 0.35 SDs. We stratified analyses based on the PAS-II score (above/below 3.7), hypothesizing that the MCII and MCIW would depend on the baseline score. To assess construct validity, we evaluated the odds of achieving the MCII in patients receiving new therapies. Results: In the overall sample, for pain and general health anchor questions, the MCIW was 0.50 and 0.55, respectively. The MCII was defined as 0.39 and 0.45, respectively, for pain and general health. The MCIW for anchor-based methods among participants with low disease activity was 1.10 (1.09/1.11 [pain/general health]), while the MCII for those with moderate-to-high disease activity was 1.09 (1.15/1.02 [pain/general health]). Distribution-based methods for 0.5 and 0.35 SD were 1.08 and 0.76, respectively, for pain and general health. There was fair-to-excellent agreement with clinically important differences in assessments of pain and disability. Patients receiving new treatments had 30% greater odds of achieving the MCII. Conclusion: The minimum important change in PAS-II score was approximately 0.5. Among participants with a moderate-to-high PAS-II score, the MCII was 1.1, and among participants with low disease activity, the MCIW was 1.1.
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