We found that CKD is associated with higher prevalence of disability across many measures of disability, including limitations in working, walking, and cognition and difficulties with ADL, instrumental ADL, and leisure and social activities, which were mostly, but not completely, attenuated by age and other comorbid conditions. Additionally, the associations differed by age: CKD was no longer statistically significantly associated with disability in younger adults after adjustment for comorbid conditions, whereas some associations among older adults generally remained robust to this adjustment. Overall, rates of disability were high and were similar to those seen in self-reported cancer, hypertension, CVD, obesity, and arthritis, even after adjustment for these comorbid conditions.
Disability is high in ESRD. Particularly with regard to limitations in ability to work, only ~15–25%, depending on age, of U.S. ESRD patients beginning dialysis treatment in 2005–2007 reported being able to work for pay and ~80% of the same patients received or had applied for Social Security disability benefits; in the year prior to starting dialysis, only ~50% of younger adults were working (12
). However, progression of earlier-stage CKD to ESRD, if it occurs at all, can be quite slow. Thus, on the population level, it may be even more important to determine the burden of disability among the estimated 26 million adults estimated to have earlier-stage CKD in the United States (24
). Although previous studies have examined particular aspects of disability, such as difficulties with ADL among an older community-dwelling cohort (3
), the national estimates presented here include a wide range of disabilities related not only to work but to other aspects of everyday living and provide a comprehensive snapshot of disability associated with CKD in the United States.
The prevalence of disability associated with CKD differed substantially by measure. It has been shown previously that the type of disease may determine the type of disability; e.g
., arthritis may be associated with greater limitations in mobility, while CVD may be associated with greater limitations in activities requiring aerobic capacity (25
). However, CKD is a disease with many manifestations—independent of associated diabetes, hypertension, or CVD—such as anemia, bone mineral metabolism, and uremia (26
), so a single effect of CKD on disability is unlikely. Indeed, we found that CKD was associated with higher prevalence of a wide range of disabilities, particularly those disabilities affected by lower functioning in both mental and physical domains, likely due to CKD manifestations and the various conditions that are frequently comorbid with CKD.
We found that younger adults generally had lower levels of disability and that the association of disability with CKD was generally attributable to comorbid conditions, prescription medications, and, possibly, associated depression (in stages 3 and 4). Older adults with CKD had higher rates of disability overall and the associations of CKD and disability were not as attenuated by comorbidity, prescription medications, or depression, suggesting that CKD exerts a stronger independent effect on disability in these individuals. For both age groups, disability was often more strongly associated with CKD in stages 1 and 2 than stages 3 and 4 after adjustment, which may be due to the increasing effects of comorbidity (and associated treatments) as CKD progresses.
There are several limitations to this study. Foremost, the cross-sectional design of this study does not allow us to establish causation since disability may be caused by CKD. However, the results do provide some evidence for causality: the graded association of CKD with disability for some measures; the magnitude of the association, even after the adjustment for other conditions that are likely associated with disability; and strong biological plausibility, in that the many pathophysiological manifestations of CKD are likely to cause mental and physical declines, leading to limitations in working, physical activities, and ADL. A second limitation is that, although sample weighting was used to estimate population prevalence, there may still be selection bias due to inclusion criteria, in that those completing the questionnaire or meeting the inclusion requirements were older and sicker than those excluded. Thus, our rates of both CKD and disability may be higher than those found in the general population. Prescription medications were self-reported and over-the-counter medications were not included. Finally, disability and comorbid diseases were both self-reported, and for the items regarding limitations only yes/no responses (versus graded or continuous responses) were recorded. Disability has a subjective component (in that a person who feels unable to perform a task will not perform the task, and thus is disabled), so self-report is an important way to measure disability. Diseases and conditions that are self-reported are likely to be under-reported; however, such misclassification would likely bias our results toward the null. Similarly, although some misclassification of CKD by single measurement of albuminuria and/or error in GFR estimation is likely, the association between CKD and disability remained robust to various definitions.
In summary, both mild and moderate CKD are associated with higher prevalence of disability in the United States, and age and other comorbid conditions account for some, but not all, of this association, particularly among older adults. The burden of disability in pre-ESRD CKD is high, and CKD is associated with disability across a wide range of measures, including activities that require both mental and physical functioning. Future work is needed to establish possible causes of and interventions to limit CKD-associated disability.