The objective of this paper was to evaluate the initial psychometric properties of the WPS-RA as a tool to estimate productivity limitations due to RA in the workplace and in household activities. In so doing, we sought to demonstrate that the WPS-RA could efficiently evaluate both the impact of the disease and clinical interventions on work outcomes in patients with RA. To this end, the discriminant validity, the responsiveness to clinical changes, and the reliability of the survey were evaluated in subjects enrolled in a clinical trial for the treatment of active RA.
OMERACT (Outcome Measures in Rheumatology) is an international, informal network of clinicians and scientists interested in outcome measurement across the spectrum of rheumatology intervention studies. OMERACT meetings 6 and 7 have highlighted the importance to patients of consideration of the impact of RA on paid and unpaid work outcomes as they represent an important component of the health and well-being of RA patients [15
]. Patient-reported outcomes (PROs) in RA have long been included in RA trials as they capture the patient's perspective of the disease process and the impact of treatments on the disease. Well-accepted PRO measures used in RA clinical trials include the HAQ-DI (which measures functional disability), the SF-36 (a generic HRQoL measure), and various pain assessments. The impact of RA on work outcomes is not currently a core component of RA clinical trials. We have thus taken initial steps to create an assessment for use in clinical trials, designed to efficiently capture the impact of RA and its treatment on work outcomes, broadly defined to include both paid and unpaid work. During the recent OMERACT 9 meeting, based on the available filter evidence (truth, discrimination, and feasibility) [31
], the WPS-RA was one of six instruments identified by the OMERACT Worker Productivity group as a possible candidate for assessing productivity changes in RA. OMERACT 9 proceedings are expected to be published this year and will fully describe the findings from the latest meeting.
In capturing work absences due to arthritis, we considered both full and partial days (that is, days of work missed and days with productivity reduced by at least half). Kessler and colleagues [32
] have used the term 'work cut back and work loss days', whereas others have used the National Health Interview Survey (NHIS) approach of disability days and partial days in bed [33
]. Still others have used work loss days and days worked but with productivity reduced by half or more [34
]. Similar subjective assessments of perceived effectiveness (or lack thereof) in performing work activities have been taken in other chronic disease states such as migraine headache and depression [35
]. Responses tend to be based on the patient's estimate of completely missed work days and of days that they worked but their productivity was reduced. Previous assessments have asked patients to estimate their productivity at work when working with symptoms and asked the patients to estimate their productivity on a scale of 0 to 100. However, it was felt that asking respondents to estimate the days in which they were less than 50% productive allowed for easier responses that were as meaningful. Lerner and Lee [38
] have noted that respondents generally underestimate time lost, so this would be a more conservative estimate of work productivity.
The discriminant validity of the WPS-RA was evaluated relative to a standard measure of physical functioning (HAQ-DI) and a validated generic HRQoL measure (SF-36). Subjects with lower physical functioning or HRQoL scores tended to have statistically greater productivity losses due to RA within and outside the home compared with subjects with higher scores; 83 of the 88 validation evaluations of the WPS-RA were statistically significant, showing that the survey has properties supportive of discriminant validity.
The known groups used to assess discriminant validity were constructed using the first and third quartiles of the instrument scores at baseline. If clinically meaningful thresholds instead of the first and third quartiles were used for physical disability or HRQoL, this would have led to a comparison of unbalanced groups for the validity analysis. However, recognized clinical thresholds were considered to assess the responsiveness of the WPS-RA, in support of the discriminant validity.
The responsiveness of the WPS-RA was tested against two meaningful clinical changes: the ACR20 and the HAQ-DI responses. At week 24, both ACR20 and HAQ-DI responders reported significant reductions in lost productivity within and outside the home, whereas non-responders reported mainly a worsening in their productivity. The effect size for productivity changes in ACR20 or HAQ-DI responders was moderate to large for six of eight WPS-RA questions (SRM = 0.48 to 1.12). In non-responders, the magnitude of change was negligible (SRM < 0.1) or small (SRM < 0.3). These results demonstrate the responsiveness of the survey, given the differences in effect size seen for responders and non-responders and the similarities in responsiveness for both criteria (ACR and HAQ).
The WPS-RA is interviewer-administered, is based on patient self-report, and has a 1-month recall period. The limitations of self-report data have been acknowledged, but previous work comparing self-report data to 'objective' data from work records and diaries supports the value of self-report data as being efficient, reasonably accurate, and often the only means by which such information can be collected [38
]. Health-related work productivity questionnaires vary in the length of recall time, and there is no consensus regarding the ideal reporting period [38
]. A 1-month recall is considered sufficient to be likely to capture events and does not overly burden respondents with too great a frequency of question-asking (as a daily diary might).
We will be undertaking future work to develop a self-administered version of the WPS-RA and to assess the utility of the instrument outside clinical trials and consequently assess criterion validity. Given the nature of the questions and the relatively good completion rates in the trial, we would expect no major differences between the self-administered and interviewer-administered versions. Criterion validity was not assessed at this point since such an assessment explores the relationship between self-report and objective productivity measures and thus determines whether responses are related substantially to actual output. This type of assessment would be more appropriate within specific workplace studies. The clinical relevance and generalizability of WPS-RA results outside of clinical trials will be assessed by defining norms for the MCID of each of the questions of the instrument.