Washington, Feb 26 : Aspirin doesn't help in preventing second stroke in 20 percent of the cases, say researchers.
The study, led by Francis M. Gengo, Pharm.D., professor of neurology in the University of Buffalo and professor of pharmacy practice in the UB School of Pharmacy and Pharmaceutical Sciences, found that up to 20 percent of patients taking aspirin to lower the risk of suffering a second cerebrovascular event do not have an antiplatelet response from aspirin, the effect thought to produce the protective effect.
"Millions of people use low-dose aspirin either for prevention of a second stroke, second heart attack or second episode of peripheral artery disease," said Gengo.
"In those three 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 preventive effect," he added.
With no definitive data on the frequency of this condition, known as aspirin resistance, physicians were left with a best guess of between 5 and 50 percent, said Gengo.
However, the researchers have now confirmed the 20 percent figure through a strictly controlled study conducted over 29 months in 653 consecutive stroke patients.
Aspirin reduces the risk of a cardiovascular event by preventing blood platelets from aggregating in the arteries and obstructing blood flow.
If blood drawn from a patient taking aspirin shows that platelets are still aggregating, that patient is diagnosed as being aspirin resistant.
If a stroke patient has a second stroke while on aspirin, the patient has experienced what is known as clinical aspirin failure.
"We've known about clinical aspirin failure for many years. 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'," said Gengo.
"That's one of the critical pieces of information provided by this paper. We looked at how frequently aspirin resistance occurred in all patients and its prevalence in patients who suffered clinical aspirin failure. What we found was, across the board, about 80 percent of the patients in our study, were aspirin sensitive -- their platelets did not aggregate in arteries -- and 20 percent were aspirin resistant.
"However, when we asked the same question of the data from patients who had a second stroke while on aspirin [clinical aspirin failures], 80 percent were aspirin resistant," he added.
The study is published in the Journal of Clinical Pharmacology's Web site as a "document of interest" and will appear in a future issue of the journal.