Kidney disease may damage the heart and vice-versa, according to two U.S. studies that bring the link to the forefront for doctors and patients.  

Kidney disease can cause progression of cardiovascular disease, while heart disease can cause progression of kidney disease, the studies involving over 50,000 patients found. 

In chronic kidney disease, the kidneys slowly lose their ability the filter blood. Unlike in heart disease where there may be symptoms such as chest pain, there are often no symptoms of kidney disease until its end stages, when dialysis or a transplant may be needed.

"Sometimes it gets picked up on early testing, but if your physician isn't looking for it then you might never know until you reach the end stages," said Dr. Dara Maker, a family physician in Toronto.

"The average patient knows their cholesterol," said Dr. Peter McCullough, preventive medicine chief at Michigan's William Beaumont Hospital, author of one of the studies in this month's issue of Archives of Internal Medicine. "The average patient has no idea of their kidney function."

In particular, doctors should check for signs of kidney disease in people with high blood pressure, diabetes or a family history.

Patients can ask for a blood test of creatinine as an estimate of how well their kidneys are working, and a urine test that looks for protein, an early sign of kidney disease.

The most important ways of preventing progression of kidney disease are by controlling blood pressure and glucose levels in people with diabetes, both of which may be accomplished without turning to drugs, Maker said.

If the kidney disease progresses, medications such as those used to control high blood pressure can be added.

Higher risk of death from both diseases 

McCullough's study of 37,000 people with an average age of 53 showed the odds of having heart disease rose as markers of kidney function declined.

The risk of death was three times higher in 2.5 years among those with both chronic kidney disease and heart disease, which was mostly attributed to heart problems.

In terms of the heart's effect on the kidneys, researchers at Tufts-New England Medical Center in Boston concluded that people diagnosed with heart disease had twice the risk of declining kidney function over the next nine years, based on their study that tracked 130,000 people.

As for why:

• Heart disease narrows arteries, including those for the kidneys.
• Compounds used in heart imaging tests may harm kidneys.
• Key signals sent from the heart and kidneys to the bone marrow for repairs may fail, McCullough suspects.

More than 30,000 Canadians need dialysis or a transplant to stay alive, according to the Kidney Foundation of Canada's website.

With files from the Associated Press