Aspirin resistance carries real risks

From San Francisco, at the International Stroke Conference

The notion that some people are impervious to the blood-thinning effects of aspirin has been debated for more than 15 years. A meta-analysis of 17 studies now bolsters the evidence for “aspirin resistance” and indicates that the trait increases vulnerability to stroke and heart attack, heightens a person’s risk of dying, and diminishes the effects of another commonly used blood thinner.

In the studies reviewed, out of 2,367 people with a history of heart problems or stroke, 618 had a blood-clotting ability that routinely overpowered aspirin’s anticlotting effects.

About 33 percent of these aspirin-resistant people experienced a stroke, heart attack, or another vascular ailment during the course of the studies. In contrast, only 15 percent of the aspirin-sensitive people experienced any of these problems, says cardiac surgeon George Krasopoulos of the Royal Brompton Hospital in London.

Moreover, 5.7 percent of the aspirin-resistant people died during the studies, compared with only 1.3 percent of those who were aspirin sensitive. Men were more likely to be resistant than women. People resistant to aspirin also seemed to get little benefit from the blood thinner clopidogrel (Plavix).

Krasopoulos, who was at Toronto General Hospital in Canada while conducting this analysis, says that previous research into aspirin resistance hadn’t controlled well for variations in the frequency of aspirin use and dosage.

The biological mechanism underlying resistance is unclear. But the new analysis suggests that people susceptible to stroke or heart attack should be tested for aspirin resistance, since having it leaves them “at high risk of getting an adverse event,” Krasopoulos says.

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