A Threshold of Meaning for Work Disability Improvement in Psoriatic Arthritis Measured by the Work Productivity and Activity Impairment Questionnaire

INTRODUCTION: The Work Productivity and Activity Impairment Specific Health Problem Questionnaire (WPAI:SHP) is used to assess the impact of an intervention on work productivity in patients with psoriatic arthritis (PsA). Unfortunately, studies reporting changes or improvements in domains of WPAI:SHP by patients with PsA have a limited threshold of meaning due to the absence of published minimal clinically important differences (MCIDs). Our objective was to determine the MCIDs for improvement in WPAI:SHP in patients with active PsA. METHODS: MCIDs for WPAI:SHP domains (presenteeism, work productivity loss, and activity impairment) were derived for patients with active PsA who were biologic naïve or TNF inhibitor (TNFi) experienced using 24-week results from two phase 3 trials (SPIRIT-P1 and SPIRIT-P2). MCIDs were derived using the anchor-based method supplemented by the distribution-based method. Anchors included achievement of the American College of Rheumatology 20 responder index (ACR20), the minimal disease activity (MDA), and the Health Assessment Questionnaire and Disability Index (HAQ-DI) MCID (improvement ≥ 0.35). Anchor validity was assessed by biserial correlation and analysis of covariance modeling against the domains. MCIDs were triangulated using the receiver operating characteristic (ROC) method supplemented by the distribution-based method. RESULTS: The analyses included 417 biologic-naïve and 363 TNFi-experienced patients. ACR20, MDA, and HAQ-DI were valid anchors. Significant differences in WPAI:SHP domain scores were observed between patients achieving ACR20, MDA, or HAQ-DI compared to patients not achieving these clinical thresholds (all P < 0.001). ROC analyses suggested that a ≥ 20% improvement in presenteeism, a 15% improvement in work productivity loss, and a 20% improvement in activity impairment represented clinically meaningful improvements in both populations. The distribution-based method supported the results. CONCLUSION: MCIDs for the presenteeism, work productivity loss, and activity impairment domains were estimated to be 20%, 15%, and 20%, respectively, in biologic-naïve or TNFi-experienced PsA populations. These results will help improve the meaningfulness of WPAI:SHP improvements reported by PsA patients. TRIAL REGISTRATION: SPIRIT-P1: NCT01695239, SPIRIT-P2: NCT02349295. FUNDING: Eli Lilly and Company.