Individual and neighborhood socioeconomic status and progressive chronic kidney disease in an elderly population: The Cardiovascular Health Study

Abstract

Few studies have focused on the association between socioeconomic status (SES) and progressive chronic kidney disease (pCKD) in an elderly population. We conducted a cohort study of 4735 Cardiovascular Health Study participants, ages 65 and older and living in 4 US communities, to examine the independent risk of pCKD associated with income, education and living in a low SES area.

pCKD was defined as creatinine elevation ⩾0.4 mg/dL (35 μmol/L) over a 4–7 year follow-up or CKD hospitalization. Area SES was characterized using measures of income, wealth, education and occupation for 1990 (corresponding to time of enrollment) US Census block groups of residence.

Age and study site-adjusted incidence rates (per 1000 person years) of pCKD by quartiles of area-level SES score, income and education showed decreasing rates with increasing SES. Cox proportional hazards models showed that living in the lowest SES area quartile, as opposed to the highest, was associated with 50% greater risk of pCKD, after adjusting for age, gender, study site, baseline creatinine, and individual-level SES. This increased risk and trend persisted after adjusting for lifestyle risk factors, diabetes and hypertension. We found no significant independent associations between pCKD and individual-level income or education (after adjusting for all other SES factors). As such, living in a low SES area is associated with greater risk of pCKD in an elderly US population.

Introduction

Chronic kidney disease (CKD) is a significant public health burden in the United States that is especially relevant in the elderly population. According to the United States Renal Data System (USRDS), the incidence rate of end-stage renal disease (ESRD) was 336 per million in 2002 (US Renal Data System, 2004) and is projected to rise to 612 per million by 2010 (U.S. Department of Health and Human Services, 2000). Incidence of ESRD increases dramatically with age, with those over age 65 exhibiting the highest rates of disease; these rates were reported in 2002 as 1308/million for those ages 65–74 and 1456/million for people 75 year and older (US Renal Data System, 2004). If left untreated, mild forms of kidney impairment can progress to ESRD. Exploring the risk factors related to kidney disease progression in an elderly population is thus essential to reducing morbidity and mortality related to CKD.

Low socioeconomic status (SES) has been found to adversely affect the risk of kidney disease, when examining individual (Krop et al., 1999; Perneger, Whelton & Klag, 1995; Rostand, Brown, Kirk, Rutsky & Dustan, 1989) or area SES (Brancati, Whittle, Whelton, Seidler & Klag, 1992; Byrne, Nedelman, & Luke, 1994; Klag et al., 1997; New York State Department of Health, 1991; Rostand, 1992; Whittle, Whelton, Seidler, & Klag, 1991; Young, Mauger, Jiang, Port, & Wolfe, 1994). In a recent study, we found a strong association between living in a low SES area and progressive chronic kidney disease (pCKD) among white middle-aged men (Stein Merkin, Coresh, Diez Roux, Taylor, & Powe, 2005). To our knowledge, that study was the first to examine CKD progression (pre-end stage) and area-level SES. Moreover, few studies have examined the associations of SES (individual or area) with kidney disease in an elderly population. To the extent that low socioeconomic position in the elderly is a marker for a history of low SES over the lifecourse, SES in the elderly may reflect cumulative exposure to conditions associated with SES and also associated with the development of CKD, such as hypertension and diabetes. Low socioeconomic position may also be a marker for access to treatment and poor control of hypertension and diabetes.

Several different dimensions of individual-level SES may be relevant to the development of CKD. This study focuses on two common measures of individual-level SES available in epidemiologic studies: income and education. Income is a measure of monetary resources and purchasing power. Income may affect access to optimum health care, medical visits and medication and may also affect a person's ability to adopt and maintain a healthy diet and physical activity. Education directly affects job attainment and income. In addition, education can influence knowledge about disease risk factors (Davis, Ahn, Fortmann & Farquhar, 1998; Winkelby, Jatulis, Frank & Fortmann, 1992) and the health care system, as well as the ability to utilize it effectively (Pincus, 1988; Sabates & Feinstein, 2006).

Area-based measures of SES may also be of special relevance to the elderly for several reasons. Place of residence in old age may be a better marker for SES over the lifecourse than measures like current income, which often decline in old age (Kaplan, Seeman, Cohen, Knudsen, & Guralnik, 1987). In addition, there may be different mechanisms through which area characteristics may be related to the risk of pCKD aside from individual-level socioeconomic position. Low socioeconomic neighborhoods have been found to be associated with a lack of physical activity facilities (Gordon-Larsen, Nelson, Page & Popkin, 2006; Powell, Slater, Chaloupka & Harper, 2006) and limited access to healthy and affordable food (Horowitz, Colson Hebert, & Lancaster, 2004; Moore & Diez Roux, 2006; Powell, Slater, Mirtcheva, Bao, & Chaloupka, 2007). In addition, studies have shown that individuals living in low SES areas are less likely to seek adequate and preventive health care (Pappas, Hadden, Kozak, & Fisher, 1997). Considering the importance of these factors with regard to controlling and avoiding the major risk factors for kidney disease, diabetes and hypertension, these neighborhood conditions may play a role in the progression of CKD. These area characteristics may be of special relevance to the elderly, since older individuals are more likely to spend time in their neighborhoods and to rely on local areas for resources and services (Robert & Li, 2001), including health care. Area characteristics may also be proxies for harmful environmental exposures including nephrotoxins (Lin, Tan, Hsu, & Yu, 2001; Steenland, Thun, Ferguson, & Port, 1990).

The goal of this study was to examine the independent associations of individual and area-level SES with pCKD in an elderly population.