The dyes used to help doctors determine if a patient's arteries are blocked may cause kidney damage for those with underlying health problems, such as early signs of kidney disease or diabetes.

A new study has found that in some patients, the two dyes often used to illuminate the arteries and blood vessels around the heart can cause enough kidney damage to increase the risk of heart attack or stroke.

It's already known that exposure to contrast agents such as iodine and barium can injure the kidneys. But patients are often told that this is only a temporary side effect.

This research suggests that such contrast-induced kidney damage may actually be more serious.

"This is not just a short-term problem, but could also have long-term implications," Dr. Madhu Natarajan of Hamilton Health Sciences told CTV News.

Each year, more than one million people in North America undergo the test, called an angiography, to determine if they have blocked arteries that need to be opened via heart bypass surgery or angioplasty.

The tests use contrast agents that enhance the contrast of structures or fluids within the body.

The dyes do not pose a risk to patients with healthy kidneys. But in a study of nearly 300 patients who showed signs of kidney disease, Canadian and American researchers found that 31 per cent of patients who received either opamidol or iodixanol contrast agents continued to have health problems a year after undergoing the test.

Thirteen per cent suffered kidney failure, a stroke or heart attack and, in some cases, death.

Individuals who developed contrast-induced kidney injuries had twice as many long-term negative health effects compared with patients whose kidneys were not damaged.

The results are published in the Clinical Journal of the American Society Nephrology.

"This is important because I think it makes us physicians, as well as patients, be more aware that this is a potential complication," Natarajan said.

Four per cent of adults have kidney damage, some of it undiagnosed, and rates of kidney problems are higher among those with Type 2 diabetes.

Therefore, if doctors give their patients a simple blood test to determine if they have kidney damage before undergoing angiography, they can be given medication before or after the test to protect their kidneys.

"We believe this therapy can help reduce the risk," Dr. Sheldon Tobe of the Heart and Stroke Foundation told CTV News. "There is still a debate among the medical community about how effective it is, but the risks of this kind of therapy are absolutely minimal."

With a report from CTV News medical correspondent Avis Favaro and producer Elizabeth St. Philip