Aspirin 'resistance' raises risk of heart trouble

NEW YORK (Reuters Health) - People who are resistant to the blood-thinning effects of aspirin are at increased risk of having a heart attack, stroke, and of dying from a pre-existing heart condition, according to a report in this week's British Medical Journal.

Millions of people take an aspirin a day to help prevent heart attacks and strokes, but doctors have known for some time that some people are less sensitive to the beneficial effects of aspirin than others.

Patients who are labeled "aspirin-resistant" have blood cells (platelets) that are not affected in the same way as those of patients who are "aspirin-sensitive."

There is currently no agreed method of accurately determining who is and isn't aspirin-resistant and the reasons why someone might be aspirin-resistant are currently a subject of debate.

Given that relatively few studies have looked at whether aspirin resistance has any impact on clinical outcome, Dr. Michael R. Buchanan, from McMaster University in Hamilton, Ontario, Canada, and colleagues reviewed 20 relevant studies involving 2,930 people who were prescribed aspirin (75 to 325 milligrams daily) as a way of preventing clots from forming in the blood. In six of the studies, subjects were taking other blood-thinning medication as well.

Aspirin resistance was identified in 28 percent of study subjects. Compared with aspirin-sensitive subjects, aspirin-resistant subjects were nearly four times more likely to experience a cardiovascular event, such as a heart attack or stroke. In particular, they found that 39 percent of aspirin-resistant patients compared to 16 percent of aspirin-sensitive patients suffered some sort of cardiovascular event.

Aspirin-resistant subjects were nearly six times more likely to die than aspirin-sensitive subjects. Moreover, there was no evidence that other anti-clotting therapies were beneficial for aspirin-resistant subjects, the report states.

"Studies need to be designed to determine the most useful test to identify aspirin resistance and identify alternative effective therapies for patients who are resistant to aspirin," Buchanan and colleagues say.

In a related editorial, Dr. Giuseppe Biondi-Zoccai, from the University of Turin, Italy, and Dr. Marzia Lotrionte, from Catholic University in Rome, comment that several questions remain: "We don't know whether aspirin resistance is a true abnormal response or whether it reflects normal variability in drug activity. We also aren't clear whether aspirin resistance has a negative prognostic effect independent of more traditional risk factors, such as diabetes or obesity."

SOURCE: British Medical Journal, online January 18, 2008.