Validity of the Workers Productivity and Activity Impairment questionnaire: Specific Health Problem (WPAI:SHP) in patients with systemic sclerosis

Objective. To evaluate the construct validity of the Workers Productivity and Impairment Activity Index: Specific Health Problem (WPAI:SHP) in Australian systemic sclerosis (SSc) patients. Methods. SSc patients, identified through the Australian Scleroderma Cohort Study database, completed the WPAI:SHP and a quality of life instrument (PROMIS-29) cross-sectionally. The construct validity of the WPAI:SHP was assessed by the correlations between the WPAI:SHP and a range of SSc health states. Non-parametric correlation, including Spearman's correlation (ρ), was used to test the validity of WPAI:SHP and ability to distinguish between different health states. Results. A total of 476 completed questionnaires was returned, equating to a response rate of 63.7%. Among those under 65 years of age, 155 patients (55.2%) were in paid employment. Employed patients had a mean (± SD) age of 56.5 (9.8) years and were predominantly female (87.3%) with limited disease subtype (75.6%). The WPAI:SHP showed construct validity based on moderate to strong correlations with health status as assessed by a range of health outcome measures including disease activity (ρ=0.34-0.39, p=0.001), physical function (ρ=0.55-0.62, p=0.001), disease severity(ρ=0.55-0.62, p=0.001), fatigue (ρ= 0.62-0.63, p=0.001), pain (ρ=0.68-0.71, p=0.001), and breathlessness (ρ=0.39-0.46, p=0.001). Furthermore, according to the effect size, the WPAI:SHP scores have a large discriminative ability (d=1.26-1.47) for distinguishing SSc patients with different health outcomes. Conclusion. The WPAI is a valid questionnaire for assessing impairments in paid employment and social activities in SSc patients, and for measuring the relative differences between SSc patients with varying health states.