Close to 20 percent of patients taking aspirin to lower the risk of suffering a second cerebrovascular event do not have an anti platelet response from aspirin, the effect expected to be protective. The University of Buffalo conducted a strictly controlled study over 29 months with 653 patients to confirm the 20 percent rate of aspirin resistance. Lead researcher, Francis Gengo, states, "Millions of people use low-dose aspirin for prevention of a second stroke, second heart attack, or second episode of peripheral artery disease. In those indications, it's crystal clear that aspirin reduces the risk of a second heart attack or stroke in most patients. But we have known for years that in some stroke and heart attack patients, aspirin has no preventative effect."

Aspirin lowers the risk of a cardiovascular event by preventing blood platelets from accumulating in the arteries and obstructing blood flow. If blood taken from a patient shows that platelets are still accumulating, then the patient is diagnosed as being aspirin resistant. If a stroke patient has a second stroke while taking aspirin, then the patient has experienced what is known as clinical aspirin failure. Gengo adds, "We're just beginning to understand clinical aspirin resistance. The major question recently has been, 'If you are aspirin resistant, does that mean you are more likely to be a clinical aspirin failure? Is one related to the other?' The answer is likely, yes."

The findings in the study need to be confirmed in a larger population study over 1-2 years, but in the meantime, Gengo suggests that clinicians could test a patient's responsiveness to aspirin early on and prescribe accordingly.

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