Study Questions Value of Test for Peripheral Artery Disease
MONDAY March 1, 2010 -- A new Scottish study raises questions about the value of both the ankle-brachial index, a test widely used to diagnose the risk of blood vessel problems in the legs, and the common practice of prescribing low-dose aspirin to reduce that risk.
The ankle-brachial index (ABI) measures blood pressure in the arm and the ankle. Conventional wisdom is that a low ABI number is a sign of peripheral artery disease, a narrowing or blocking of arteries in the legs that can lead to clotting problems in the lower limbs, heart disease or stroke. Aspirin reduces the chances of such blood clots but can raise the risk of bleeding episodes.
The Scottish study, which followed nearly 29,000 cardiovascular-healthy men and women aged 50 to 75 for nearly 10 years and assigned 3,350 to a test of aspirin therapy on the basis of ABI test results, found no decreased incidence of blood vessel problems -- but a higher incidence of major bleeding incidents -- in the aspirin group.
To Dr. F. Gerald R. Fowkes, a professor of epidemiology at the University of Edinburgh and lead author of the report in the March 3 issue of the Journal of the American Medical Association, the results show that routine use of the ABI test for screening in people without risk factors for cardiovascular disease might not be warranted.
"That there was no difference in the two groups is partly due to the fact that when you screen people who in general feel quite healthy, some of those individuals don't continue taking aspirin because they feel quite well," Fowkes said.
So ABI test screening might be reserved for people with known risk factors, he said. As a general screening tool, "our trial would suggest that in clinical practice one would have to approach this with caution," Fowkes said. "If we are going to use the ABI to screen individuals, it is better to go to individuals who already are at higher risk."
Prescribing aspirin to otherwise healthy people should also be approached with caution, he said.
"With known cardiovascular disease, there is no doubt that there are people who should be taking aspirin," Fowkes said. "For individuals in any given population who have no evidence of cardiovascular disease, the evidence is still uncertain. The trial suggests that any benefit that aspirin gives may be counterbalanced by an increase in bleeding risk, and therefore that many doctors should not recommend that aspirin be given to healthy people."
Any recommendation for aspirin should be accompanied by a thorough explanation, Fowkes said. "We expected that people with a low ABI would get some reduction in cardiovascular events from aspirin," he said. "We were quite surprised to find the trial results to be negative. Perhaps these people were less likely to stick to the drug. There is a bit of an information gap. People don't understand why a test done at the ankle should tell a doctor something about the risk of heart attack and stroke."
Dr. Jeffrey S. Berger, director of cardiovascular thrombosis at New York University, who wrote an accompanying editorial, agreed that "the use of the ABI in a population without established cardiovascular risk needs to be reconsidered."
"So many people are using aspirin to prevent a first heart attack or stroke that we need to see whether the benefit outweighs the risk," Berger added.
The issue should be discussed by doctors and patients, he said. "It is up to the patient to weigh whether or not to take aspirin," Berger said. "Not every bleed is the same. Some might think that a heart attack is the big risk, others might think that a bleed might be worse. A bleed might be in the brain. Both physician and patient should discuss it."
More studies are needed to clarify the use of ABI testing and the therapy that might be prescribed when the test indicates risk, Berger said.
"About 5 to 10 percent of the population have abnormal ABIs," he said. "What should we be doing for them? That we don't know. The data is very weak. We need studies to determine whether if we find an abnormal ABI and modify treatment based on it, will that improve patient outcome."
The why and how of the ankle-brachial index are described by the Vascular Disease Foundation.
Posted: March 2010