Disability, as measured by reduced physical functioning (1,2), frailty (3,4), and decreased cognitive functioning (5-8), is commonly reported among people with chronic kidney disease (CKD) and end-stage renal disease. Socioeconomic disparities in both the development and progression of CKD have also been well described (9-12). Socioeconomic factors have been associated with disability in the general population, but this relationship has not been studied among people with CKD (13). Identification of socioeconomic status as an independent risk factor for disability in people with CKD, beyond the high risk conferred by CKD and its risk factors, would provide a means for early identification of those at risk and, possibly, intervention.

Race/ethnicity and other factors that contribute to socioeconomic status (SES), including income and education, could affect both CKD and disability through multiple pathways (Figure). Environmental and personal factors could lead to both CKD and disability; risk factors for CKD and disability, including diabetes and hypertension, could also be heavily influenced by the same factors. Such risk factors could lead directly to disability or could lead to CKD, which in turn could lead to disability through complications of CKD. Additionally, older age is strongly associated with both CKD and disability and could lead to disability directly or through increased presence and severity of comorbid conditions or increased use of medications with age. Finally, disability could affect modifiable sociodemographic factors, particularly income, if the disability interferes with work (14).

Our primary objective was to establish whether a socioeconomic (defined as education level and household income) disparity exists in the prevalence of self-reported disability among noninstitutionalized US adults with CKD, independent of the unique complexity of this disease, which involves many coexisting factors that may contribute to disability. Our secondary objective was to compare these associations with the association of race/ethnicity and disability. We also explored whether age and the severity of CKD modified the association of disability with SES and compared our findings in people with CKD and people without CKD.

Disability, among a variety of domains, is prevalent in the US adult population with CKD. Lower educational attainment and lower income were strongly and consistently associated with higher prevalence of disability — that is, among all measures and independent of the other sociodemographic and clinical factors that we measured. Even in the presence of other conditions that can cause disability (eg, diabetes, CVD, cancer, arthritis), comorbid CKD confers a higher likelihood of disability, and this excess prevalence is greatest among people with lower educational attainment. Associations between CKD and SES (9,10,12,17) and between SES and disability (18) have been examined previously. However, to our knowledge, this is the first study to examine SES as a potential risk factor for disability among people with CKD, independent of minority race/ethnicity, older age, and the myriad comorbid conditions associated with prevalence of CKD and disability.

We found that lower educational attainment and lower income, measured as either total income or as income relative to the federal poverty level (http://www.census.gov/hhes/www/poverty/data/threshld/index.html), were inversely related to all measures of disability. This linear association reinforces the presence of a powerful SES health gradient in which health outcomes are worse with lower income, regardless of whether income is above or below the poverty level (13,14,19). Additionally, in older adults, educational attainment was strongly associated with disability, as was lower income, suggesting the association is likely to be in the direction of SES to disability (vs disability to SES), because people with disability may have a subsequent loss in income but not in educational attainment.

For people with CKD, lower SES may lead to disability through poor control of the conditions that cause CKD, particularly hypertension and diabetes, because of the person's lack of understanding of the disease or the resources needed for adequate self-management and treatment. For example, poor blood pressure control, which is common among people with CKD (20,21), could lead to disability not just through stroke but through other systemic effects of hypertension (22). In diabetes-associated CKD, poor glycemic control may also lead to greater risk of disability; one study suggests that retinopathy and neuropathy, sequelae of poor glycemic control, were strong predictors of disability in African Americans with diabetes (23). We found that poor glycemic control did not change the association of education and income with disability in people with CKD and diabetes. Additionally, we showed that people whose CKD was complicated by diabetes and hypertension had higher rates of disability than those with diabetes and hypertension but no CKD, suggesting that CKD has an independent effect on disability prevalence beyond that conferred by these 2 etiologic factors.

Environmental factors, including neighborhood effects (11,24), could also contribute to the observed disparities in disability among people with CKD. Personal psychosocial issues such as depression, which is prevalent among people with CKD (25) and less likely to be successfully treated in people with lower SES (26), may also lead to disability. Lower education level is associated with limited employment opportunities, and available jobs may require intense manual labor or carry increased risk of injury. This situation may partially explain why education affects LEM disability only in the younger (working) population.

Adjustment for comorbidity and medication did not change the effect of education and income on disability, which suggests that these factors are unlikely to explain the associations. Inflammation, anemia, and level of physical activity also did not explain the association. In subgroup analyses, we found that income was more strongly associated with disability in younger (20-64 y) people. This supports the "reverse causality" scenario of decreased physical and mental functioning leading to lower income in people of working age through unemployment, early retirement, or disability. In general, there was little evidence that other sociodemographic factors modified the effect.

In contrast to the associations seen with SES, adjustment for education and income nullified most of the associations of race/ethnicity with disability. The lack of effect of minority race/ethnicity may be due to lack of power in our study; such disparities in disability have been noted in other populations (27,28). However, this lack of effect may also suggest that biological differences that would lead to disparities in disability by race/ethnicity are unlikely and that social status, which is related to race/ethnicity for many, is a more important predictor of disability, at least among people with CKD.

Our study has several limitations. First, because our study was cross-sectional, we cannot establish causality, despite strong indications of causality, including degree of association and consistency of association among measures. Some misclassification of disability and comorbid conditions is possible because these conditions are self-reported. Misclassification of CKD is also possible because of measurement error. The cross-sectional design did not allow assessment of lifetime socioeconomic status, which may be a better predictor of health outcomes than current income (29,30). Also, although CKD is generally a silent disease with few symptoms in its early phases, certain complications of CKD, such as anemia and uremia, could lead directly to disability. We were not able to examine the mechanism by which complications of early-stage CKD lead directly to disability because such complications were rare in our sample. Single assessments of eGFR and albuminuria likely led to misclassification of some participants as having chronic disease; however, any misclassification bias created would be toward the null. Additionally, Mexican Americans were treated as "not African American" in the calculation of eGFR, which may misclassify them, because the equations were developed for primarily white and black non-Hispanic populations.

Despite these limitations, identifying a socioeconomic disparity in disability among people with CKD similar to disparities seen in non-CKD populations may be useful, because both disability and lower SES can worsen CKD outcomes. Because the disparities we identified were independent of age, race/ethnicity, and common comorbid conditions, our findings provide support for a prospective study of the relationship between socioeconomic status and subsequent development of disability among people with CKD. If the associations reported here were confirmed in a longitudinal study with appropriate causal inference, people with lower educational attainment or lower income who are at risk for CKD could be targeted for earlier identification and intervention to limit the development of disability and possible further loss of income due to disability. Further exploration of the mechanisms by which lower SES is associated with greater risk of disability among people with CKD is needed to identify effective interventions.