Participants
The Cardiovascular Heath Study (CHS) is a prospective cohort study of community-dwelling persons 65 years of age and older designed to examine the epidemiology of cardiovascular disease in older adults.10 Between 1989 and 1990, the CHS recruited 5,201 Medicare-eligible participants from Forsyth County, NC; Sacramento County, CA; Washington County, MD; and Pittsburgh, PA.11 Between 1992 and 1993, the study recruited an additional 687 African-American participants. CHS exclusion criteria included: institutionalization, inability to provide informed consent, intention to move outside the enrollment area within 3 years, use of a wheelchair within the home, hospice care, or current chemotherapy or radiation for cancer. At the time of study entry, extensive clinical data were collected, including demographics, comorbid conditions, biochemical data, and prevalent cardiovascular risk factors. Participants were followed prospectively for cardiovascular events (coronary artery disease, angina, congestive heart failure, stroke, claudication), hospitalizations, and death by contact every 6 months.12,13
To focus on the competing risks of death versus progression of chronic kidney disease, we selected only those CHS participants with a Modification of Diet in Renal Disease estimated glomerular filtration rate (MDRD eGFR) < 60 ml/min per 1.73 m2 at baseline. We further excluded participants if they had no baseline serum creatinine measurement, prevalent ESRD requiring dialysis therapy, or the cause of death could not be determined.
Outcomes
The primary outcomes of interest were ESRD (defined as renal failure requiring renal replacement therapy) and death; CV and non-CV death were secondary outcomes of interest. To ascertain ESRD, the CHS cohort was linked to the United States Renal Data System (USRDS) in 2005 (which included data through March 31, 2003). In addition, a chart review was performed to determine which CHS participants met criteria for ESRD yet elected not to initiate dialysis, died prior to receiving dialysis, or died prior to being enrolled in the USRDS (e.g., initiated dialysis in the hospital and subsequently withdrew dialysis). To identify these potential CHS participants with ESRD who were not included in the USRDS, charts of CHS participants were reviewed if the participant had a procedure code for dialysis, discharge diagnoses suggesting ESRD (e.g., renal failure, ESRD, dialysis, or renal transplant), or the cause of death was renal failure or failure to thrive. The charts were reviewed by one investigator (LF), and potential cases were reviewed by the CHS renal working group for concurrence.
CV death was defined as death due to cardiovascular, cerebrovascular, atherosclerotic, or other vascular causes, and was adjudicated by the CHS Events Subcommittee by a protocol previously described in detail;12 any death other than those mentioned above was considered to be a non-CV death (e.g., cancer, dementia, pulmonary disease, or infection). In the CHS study, death was determined by examining the Social Security Death Index, Medicare claims data, local obituaries, and proxy report. The cause of death was adjudicated by committee and was based on review of medical records, death certificates, autopsy reports, and descriptions by the proxy.12,14 Participants were followed until the time of a first event or until March 31, 2003, whichever came first.
Clinical Assessment and Measurement
All methods employed to asses the baseline characteristics of the CHS cohort have been described elsewhere.10,13,15 Demographic factors (age, gender, race), health behaviors (smoking), co-existing illnesses (diabetes and hypertension), physical exam findings (body mass index, systolic and diastolic blood pressure), and laboratory values (total cholesterol, creatinine and glucose) were ascertained at study entry. Diabetes was defined by the use of a medication for diabetes or fasting glucose level ≥ 126 mg/dl. Hypertension was defined by systolic pressure ≥ 140 mmHg, diastolic pressure ≥ 90 mm Hg, or the use of antihypertensive medication(s). Prevalent coronary heart disease (CHD) and prevalent congestive heart failure (CHF) were defined using participant self-report, medical record review, findings from the baseline physical examination, and physician questionnaires.13 Smoking was classified as never, former, or current. Serum creatinine levels were measured using the Kodak Ektachem 700 Analyzer (Eastman Kodak, Rochester, New York), a colorimetric method. Serum creatinine was calibrated to the Cleveland Clinic Laboratory using indirect calibration to the NHANES III data.16 Creatinine-based estimated GFR was calculated using the four-variable Modification of Diet in Renal Disease (MDRD) equation,.17
Statistical Analyses
Baseline characteristics by categories of MDRD eGFR (eGFR 45 to < 60 or <45 ml/min per 1.73 m2) were described as means, medians, or proportions. Unadjusted incidence rates for progression to ESRD and all-cause mortality (further categorized as CV or non-CV related) were calculated per 100 person-years and also described with respect to eGFR categories. A standard Cox proportional hazards model analysis was not felt to be adequate in the presence of competing risks because the cause-specific Cox model treats competing risks of the event of interest as censored observations, and the cause-specific hazard function does not have a direct interpretation in terms of survival probability. Therefore, a competing risk model was used to examine risk factors for ESRD as compared with all-cause mortality. The variables included in the competing risk model were age, gender, race, BMI (<18.5, 18.5–24.9, 25–29.9, or ≥30), hypertension, diabetes, cardiovascular disease, heart failure, tobacco use (never, former, or current), eGFR, and total cholesterol (<200, 200–239, or ≥240 mg/dl). These variables were selected for inclusion in the analysis because of their potential association with the risk of ESRD and/or death and their availability in usual clinical practice (thereby allowing for inference by clinicians). First, we computed the cumulative incidence function (CIF) of ESRD over time. At time t, the CIF defined the probability of having ESRD by time t while other participants had experienced a death (CV or non-CV). We used the method proposed by Fine and Gray, which is based on the proportional hazards model and models directly the effect of covariates on the CIF for competing risk data.18 Our model distinguished between participants who were still alive and those who had already failed from competing causes and allowed for direct inference regarding the effects of covariates on the CIF. The cumulative incidence of ESRD in the presence of CV and non-CV mortality as competing risks was calculated similarly to the Kaplan-Meier method, except in the competing risk method participants who died were counted as events when calculating the event-free survival and only participants who were truly alive were considered at risk for ESRD. Similarly, the cumulative incidences of CV and non-CV mortality in the presence of ESRD as a competing risk were calculated. All analyses were performed using R 2.9.1 software (R Foundation for Statistical Computing http://www.R-project.org) and SPSS statistical software (release 15.0.1.1, Chicago, IL).
Investigational Review Board approval for the data collection procedures of CHS was obtained at each of the four clinical sites and at the Data Coordinating Center (University of Washington).