In these recent data from a US national cohort of subjects with RA, older age and the work-related factors of lower income from employment, fewer hours worked per week, and preference not to work predicted work disability in the main analysis using multivariable regression. Four reviews of the RA work disability literature (1
) found that greater functional limitation and physically demanding jobs were prominent risk factors in almost all studies. The results from our main analysis therefore differ from most prior studies in that neither of these factors predicted work disability.
The significance of functional limitation and RA global severity in the hierarchical regression model was reduced when work-related factors were entered, suggesting their effect was mediated by the characteristics of subjects' work. This is surprising given the strength of functional limitation, or other disease variables, in prior studies and raises the possibility that disease factors are now less prominent due to improved disease status.
However, there are other possible explanations. The first is our use of a work disability definition that includes any work cessation, irrespective of its cause, e.g., whether attributed to RA or to early retirement. We used this definition for 3 reasons. First, we wished to make our results comparable with other US studies, and 2 prominent studies had defined work disability as any work cessation (3
). Disease factors were significant predictors of work disability in both studies. A third prominent study used RA-attributed work cessation as the outcome (4
), and when we used this outcome in our study, disease factors remained significant in the final regression model. A second reason for using the any work cessation definition is that although a decision to stop working is likely to be influenced by a person's health (Yelin E: personal communication), this influence may be partial and not recognized. The any work cessation definition includes work cessation partly, as well as mostly, due to RA. Finally, subjects who cite RA as the cause of their work cessation may have more severe disease. So RA-attributed work disability, as well as definitions such as self-reported disability or Social Security disability (14
), may be defined in part by severe disease. It could be expected then that disease factors would predict this level of work disability.
Another possible explanation for the lack of significance of disease factors is that functional limitation and RA global severity are both self-reported measures and therefore subject to response set bias, e.g., influenced by adaptation to disease. Studies using rheumatologist-reported clinical indicators like number of disease flares showed such indicators have their greatest impact on employment early in the disease process (5
). The mean disease duration of our sample was 13 years, and this could explain the diminished effect of disease variables in our study. However, Eberhardt et al found that self-reported functional limitation, measured by the HAQ, was a strong predictor of RA work disability at all lengths of disease duration (38
). Furthermore, the importance of clinical indicators in predicting work disability was diminished in a cohort recruited in 1998 compared with a cohort recruited in 1987 (12
), which could indicate improved disease status.
Results of the recursive partition analyses revealed that disease factors have some continuing influence. Disease duration was ranked as the second most important factor in the random forest analysis, and functional limitation and RA global severity classified work disability cases among older subjects and were ranked fourth and fifth in importance.
The lack of predictive capacity of greater job physical demand in our study could be due to our sample's relatively long mean disease duration. Subjects with the most demanding jobs may already have stopped working and therefore were not eligible for our sample. The study by Eberhardt et al suggested that the effect of heavy manual jobs was greatest in the early years of disease (38
). The physical demand of jobs in the US has been declining for some time, however, and is a possible explanation of reduced prevalence of RA work disability (16
Older age was the most prominent predictor of work disability. It was significant in the regression analysis, and ranked first in importance. An interaction between age and RA disease variables emerged through recursive partitioning analysis, showing that disease severity was an important determinant of work disability in older subjects. It is not clear why disease factors were so important among older subjects. We considered the possibility of treatment disparity (39
); however, equal portions of older and younger subjects used an anti-TNF agent.
Other significant predictors of work disability in the regression analysis were lower income, fewer hours worked per week, and personal preference not to work versus to work either full or part time. Many studies (40
) concur in finding that the structure of work and reduced number of hours worked increases risk for work disability. Reisine et al found that subjects with RA who worked <30 hours/week were more apt to stop working and suggested this could indicate lower work commitment (26
). In our study, number of hours worked was ranked third in importance in the random forest analysis.
Reisine et al were the first to assess preference to work, and they also found it predicted which subjects became work disabled (26
). The causal modeling analyses indicated that neither the conditions of subjects' jobs nor their degree of disease severity affected their preference to work. Although preference not to work was significant in the regression analysis, it was ranked low in importance by the random forest analysis, probably because few subjects preferred not to work.
Our data suggest that older patients may be particularly vulnerable to the effects of disease on employment. Employment maintenance among older workers is an increasingly important goal, both for individuals and society (11
). Our results suggest that reducing employment hours is not an effective employment maintenance strategy, most likely due to its impact on income. Two prior studies (5
) found self-employment was associated with work retention. However, in our study it was not significant in the regression analysis. The classification tree analysis may offer an explanation of this discrepancy; self-employment predicted work disability only when it was combined with low job physical demand. Low job physical demand could be a marker for minimal work effort capacity, and if so, self-employment is effective only when the individual is capable of making a substantial work effort.
Strengths of our study are availability and use of recent data from a US national cohort of patients with rheumatologist-diagnosed RA and detailed assessment of employment information. Furthermore, we used several different methods of analysis, and this has provided a more complete picture of the relative importance of the risk factors, as well as information about the basis of work preference.
The generalizability of our findings may be restricted because our sample is not representative of all persons with RA. One-third of NDB participants are from pharmaceutical registries. Registry subjects were more likely to be male (23% versus 16%), have shorter disease duration (12 versus 14 years), have greater functional limitation (mean HAQ score 0.9 versus 0.8), and less likely to have a professional/managerial job (38% versus 46%) than non-registry subjects. However, the difference in the proportion of registry subjects among cases and controls (34% versus 31%) was quite small. About 12% of NDB participants were not eligible for this study because they fill out short questionnaires containing no questions about employment. These individuals were less likely to be white (84% versus 89%), married (68% versus 73%), or have education beyond high school (45% versus 61%) than subjects. Participants (n = 3,680) who supplied data on an irregular basis were 1 year younger, more likely to be male (22% versus 18%) or nonwhite (90% versus 92%), and had 1 year shorter disease duration than 1,088 eligible participants. Our sample was not population based. Subjects were more often white and had higher educational attainment than the US population. Consequently, persons with physically demanding jobs were probably under-represented in our sample.
In summary, the results of these recent data from a US national cohort differ from previous studies in that neither disease factors nor job physical demand were significant risk factors for RA work disability in the main analysis. Disease factors were important risk factors in alternative analyses, however, especially among older subjects. Older age, lower income from employment, fewer hours worked per week, and preference not to work were the significant risk factors for work disability in the main analysis.