Discriminant validity, responsiveness and reliability of the arthritis-specific Work Productivity Survey assessing workplace and household productivity in patients with psoriatic arthritis

Abstract

Introduction: The novel arthritis-specific Work Productivity Survey (WPS) was developed to estimate patient productivity limitations associated with arthritis within and outside the home, which is an unmet need in psoriatic arthritis (PsA). The WPS has been validated in rheumatoid arthritis. This report assesses the discriminant validity, responsiveness and reliability of the WPS in adult-onset PsA.

Methods: Psychometric properties were assessed using data from the RAPID-PsA trial (NCT01087788) investigating certolizumab pegol (CZP) efficacy and safety in PsA. WPS was completed at baseline and every 4 weeks until Week 24. Validity was evaluated at baseline via known-groups defined using first and third quartiles of patients' Disease Activity Score 28 based on C-reactive protein (DAS28(CRP)), Health Assessment Questionnaire-Disability Index (HAQ-DI), Short Form-36 (SF-36) items and PsA Quality of Life (PsAQoL) scores. Responsiveness and reliability were assessed by comparing WPS mean changes at Week 12 in American College of Rheumatology 20% improvement criteria (ACR20) or HAQ-DI Minimal Clinically Important Difference (MCID) 0.3 responders versus non-responders, as well as using standardized response means (SRM). All comparisons were conducted on the observed cases in the Randomized Set, regardless of the randomization group, using a non-parametric bootstrap-t method.

Results: Compared with patients with a better health state, patients with a worse health state had on average 2 to 6 times more household work days lost, more days with reduced household productivity, more days missed of family/social/leisure activities, more days with outside help hired and a significantly higher interference of arthritis per month. Among employed patients, those with a worse health state had 2 to 4 times more workplace days lost, more days with patient workplace productivity reduced, and a significantly higher interference of arthritis on patient workplace productivity versus patients with a better health state. WPS was also responsive to clinical changes, with responders having significantly larger improvements at Week 12 in WPS scores versus non-responders. The effect sizes for changes in productivity in ACR20 or HAQ-DI MCID responders were moderate (0.5 < SRM < 0.8) or small.

Conclusions: These analyses demonstrate the validity, responsiveness and reliability of the WPS, as an instrument for the measurement of patient productivity within and outside the home in an adult-onset PsA population.

Figures

Figure 1

Mean changes from baseline in the Work Productivity Survey by American College of Rheumatology 20% improvement criteria clinical response at week 12. Change from baseline in the Work Productivity Survey by American College of Rheumatology 20% improvement criteria (ACR20) clinical response at week 12 (randomized set, observed cases). **P ≤ 0.001, *P < 0.01 responders versus nonresponders; P-values were obtained using the nonparametric bootstrap t method. Rate of interference is a score on a scale of 0 to 10 points (0 = no interference and 10 = complete interference). WP, work productivity.

Figure 2

Effect size of mean changes from baseline in the Work Productivity Survey at week 12 (randomized set, observed cases). Effect size (standardized response mean (SRM)) of mean changes from baseline in the Work Productivity Survey (WPS) are presented by the (a) American College of Rheumatology 20% improvement criteria (ACR20) clinical response, (b) American College of Rheumatology 50% improvement criteria (ACR50) clinical response, and (c) Health Assessment Questionnaire – Disability Index (HAQ-DI) minimal clinically important difference (MCID of 0.3) clinical response. SRM (absolute values) thresholds: small, from 0.2 to 0.5 (below the dashed line); moderate, from 0.5 to 0.8 (between the two lines); and large, >0.8 (above the solid line).