Does aspirin work equally well for everyone?

We all know that aspirin can help prevent a heart attack or stroke, but does aspirin work equally well for everyone? A growing amount of medical evidence says no. One reason emerging to explain this difference is that some people fail to achieve an adequate response to the drug.

A new study adds weight to that theory, showing that lack of response to aspirin therapy greatly increases the risk of serious cardiovascular complications. The research results were presented at the annual meeting of the American Association of Clinical Chemistry (AACC), taking place this week in Orlando, Florida.

The study, conducted at the University of Hong Kong, found that fully 25 percent of the 422 patients were aspirin nonresponders. The subjects were followed for an average of about 11 months to determine how their response to aspirin affected the risk of cardiac events. Researchers found that patients with an inadequate response to aspirin were more than twice as likely to have a heart attack or stroke, be admitted to the hospital for chest pain, or die, compared to patients who had a therapeutic response to aspirin.

"These are important findings," said Alan Maisel, professor of medicine at the University of California, San Diego, and a researcher in the field of cardiac outcomes. "It adds to the growing amount of evidence documenting the consequences of aspirin failure, and shows that nonresponsive patients are indeed at significantly greater risk for potentially life-threatening events."

Several factors were associated with a poor response to aspirin. Most notably, women were much more likely to be aspirin nonresponders. Another factor was taking a lower dose of aspirin.

"The finding that aspirin nonresponse is more common among women is particularly interesting in light of information released recently from the Women's Health Study," said Daniel Simon, MD, associate director of interventional cardiology at Brigham and Women's Hospital in Boston. "That study indicated that women often do not benefit from aspirin to the same extent that men do. This new research points to one possible explanation for the disparity, namely, that women are less likely to achieve a therapeutic response to aspirin. This presents an intriguing direction for further exploration."

Aspirin response in the AACC study was evaluated using the VerifyNow(TM) System, a rapid, easy to perform blood test that assesses platelet function. Inhibition of platelet function is how aspirin exerts its cardioprotective effects. The VerifyNow test gives one, easy to understand result that indicates if a person is responding effectively to aspirin or if they are not.

"One of the major implications of this research is that we need to be paying more attention to the dose of aspirin that patients are taking," Dr. Maisel added. "For patients who do respond to low doses, that's great. We can confidently keep them on that dose without risking bleeding complications. But for patients who don't respond, higher doses are warranted to ensure that they are adequately protected. Testing aspirin response is a pivotal element of this clinical decision-making and now for the first time we have a practical, office-based test that has been shown to correlate with clinical outcomes."