Preclinical Kidney Disease in Elderly People

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The summary below is from the full report titled "Cystatin C and Prognosis for Cardiovascular and Kidney Outcomes in Elderly Persons without Chronic Kidney Disease." It is in the 15 August 2006 issue of Annals of Internal Medicine (volume 145, pages 237-246). The authors are M.G. Shlipak, R. Katz, M.J. Sarnak, L.F. Fried, A.B. Newman, C. Stehman-Breen, S.L. Seliger, B. Kestenbaum, B. Psaty, R.P. Tracy, and D.S. Siscovick.

What is the problem and what is known about it so far?

The kidneys remove waste and water from the blood. In chronic kidney disease, kidney function worsens over time until the kidneys stop working and dialysis is needed. Dialysis uses machines to filter the blood. Caring for kidney disease involves controlling diet, controlling blood pressure, and taking medications to delay heart complications and the need for dialysis. Unfortunately, many patients do not know that they have chronic kidney disease and do not get care to delay complications. Glomerular filtration rate (GFR) is a measure of kidney function. Chronic kidney disease is defined by a GFR less than 60 mL/min per 1.73 m². Cystatin C is a substance that increases in the blood as kidney function worsens. Some researchers believe that there is a stage between completely normal kidney function and chronic kidney disease called "preclinical kidney disease" that also puts people at risk for bad health outcomes. One way to define preclinical kidney disease is by high cystatin C levels in patients with normal GFR. If preclinical kidney disease puts people at risk for bad health outcomes, treatments for this early stage of kidney disease might improve those outcomes.

Why did the researchers do this particular study?

To compare the risk for cardiovascular disease (heart attack, heart failure, and stroke) and death in elderly people with normal kidney function, preclinical kidney disease, and chronic kidney disease.

Who was studied?

4,663 people who were participating in the Cardiovascular Health Study, a large study of cardiovascular disease in elderly persons.

How was the study done?

The researchers classified patients as having normal kidney function, preclinical kidney disease, or chronic kidney disease on the basis of the results of blood tests for cystatin C and an equation that estimates GFR using patient characteristics and a blood test. The researchers then contacted patients by telephone every 6 months and in person every year for up to 9 years to collect information about cardiovascular disease events, such as heart failure, stroke, and heart attack. They determined whether a patient died by contacting patients' households and reviewing medical records and death certificates.

What did the researchers find?

Among the study participants, 39% had normal kidney function, 39% had preclinical kidney disease, and 22% had chronic kidney disease. Patients with preclinical kidney disease had a risk for cardiovascular disease and death that was less than that of patients with chronic kidney disease but was higher than that of patients with normal kidney function.

What were the limitations of the study?

The study does not tell us whether treating patients with preclinical kidney disease would improve their health outcomes.

What are the implications of the study?

Even in its preclinical stages, kidney disease increases the risk for cardiovascular disease and death in elderly people. Research is needed to determine whether identifying and treating individuals with preclinical kidney disease can improve their health outcomes.