My Heart Belongs to Kidney Disease

Plugging the results of a common blood test into a simple equation can warn people that their kidneys are ailing, a condition that dramatically increases the risk of heart attacks and strokes.

When people think of heart attacks, rarely do their minds stray to their kidneys. But heart disease can assail these organs, and impaired kidneys can in turn make heart disease worse. To educate the public and doctors about kidneys' unrecognized influence on the cardiovascular system, the National Institute of Diabetes and Digestive and Kidney Diseases is spreading the word about a simple formula that could allow doctors to diagnose kidney disease--and potentially prevent heart attacks and stroke.

Heart disease announces its presence with a thumping chest. But kidneys, those bean-shaped organs that extract waste products from blood and regulate our water balance, lose their filtering ability slowly and silently. No obvious symptoms, such as soreness while urinating, alert those who are on the road to total kidney collapse. Once kidneys slip to 5% to 10% of their original muster, they fail, sending 400,000 unsuspecting victims to dialysis machines or kidney transplants every year. Half of dialysis patients die of heart attacks or strokes. But clues to kidney health exist--the three biggest risk factors for disease are high blood pressure, diabetes, and having a family member who suffered kidney failure. Age also slows down the kidneys: The average 70-year-old has half the kidney function of a 30-year-old.

But kidney function can be tricky to measure. Monitoring used to require taking urine samples continuously, a cumbersome process fraught with error. Then doctors started measuring a metabolite called creatinine. Muscles produce the compound as they perform their daily routines, and kidneys are supposed to filter the metabolite out of the blood. A buildup of creatinine suggests that the kidneys are not doing their job.

The problem with creatinine is that its concentration doesn't mean the same thing for everyone. For example, strapping young men have more muscle than women--even athletic ones--and the elderly, so the "normal" range is broad. And large dives in filtering ability often translate into small gains in creatinine concentrations--differences that physicians can mistakenly disregard as being insignificant. "Someone may have lost 60% of [his or her] filtering ability and will still be in the normal range," says nephrologist Thomas Hostetter of the National Kidney Disease Education Program (NKDEP) in Bethesda, Maryland.

To put everyone's creatinine concentrations into the same light, nephrologists led by Andrew Levey of the Tufts–New England Medical Center in Boston determined how a variety of factors influence the amount of creatinine that spills into the blood. They then developed an equation that takes into account a patient's age, sex, and race and spits out a single number that reflects the rate at which the kidneys filter blood, or the glomerular filtration rate (GFR). Plugging the results of a creatinine blood test into this formula reveals those people whose kidneys are filtering suboptimally.

Using such calculations, nephrologist Alan Go at Kaiser Permanente of Northern California in Oakland and colleagues examined 1 million patients enlisted in Kaiser's health organization and found that once the GFR dropped below half the optimum rate, an individual's risk of dying or of experiencing some type of cardiac event shot up by 20% to 40% compared to people of the same age and similar cardiovascular health who have no filtering loss. Patients whose kidneys were half again as slow were about 300 times as likely to have heart attacks, revealing that ill kidneys compound the problem of a sick cardiovascular system.

In a second study of 14,500 people, cardiologist Marc Pfeffer of Brigham and Women's Hospital in Boston and colleagues also found that the lower the kidney function, the higher the risk of death 3 years after a heart attack. The filtering in kidneys occurs in tiny blood vessels, which are just as spoiled by a faulty cardiovascular system as are other vessels. Bad filtering in turn leads to junk in the bloodstream that can irritate already sensitive arteries in other parts of the body, says Go. Such a scenario might explain why hampered kidneys go organ-in-organ with heart disease.

Pfeffer's group also found that those with the worst kidneys were the least likely to be taking any medications for their bad cardiovascular health--a counterintuitive phenomenon termed therapeutic nihilism. Researchers don't understand where such nihilism comes from but suspect that patients or their physicians are "physically throwing in the towel," says Hostetter, because of the advanced degree of illness. Whether cardiovascular drugs would help the limpest kidneys is not clear, says Hostetter. Although some small studies suggest that drugs might improve health, most cardiovascular studies deny patients with kidney problems from participating.

Kidney disease is a serious problem from a public health standpoint. "For people with risk factors and low GFR, physicians need to be really aggressive in their treatment," says Go. "And we need to systematically educate and inform those who are at risk." To that end, researchers at NKDEP launched a program to assess how much people in the highest risk group and physicians know about kidney disease. Volunteers toured four cities to survey people about their risks of kidney disease. Of those questioned, 45% had at least one risk factor, but fewer than 15% were aware that this could translate into kidney problems. "People thought they'd have some symptoms, like pain urinating, but kidney disease is silent," says nephrologist Janice Lea of Emory University in Atlanta, Georgia.

NKDEP then disseminated facts about kidney disease through newspapers, radio, and television and held health fairs at churches. The preliminary results were encouraging enough that NKDEP has launched the program nationwide. "Every city that has a kidney disease association has been asked to bring up this information," Lea says.

In the meantime, Hostetter says that clinical labs should be converting creatinine concentrations to GFR: "They have the information in their system to do that." Perhaps the thumping of NKDEP's drum in all areas of public health will let us hear the silent killer in our kidneys.

Mary Beckman writes from southeastern Idaho, where her blood pressure jumped after the election.