In what is believed to be the first direct comparison of blood cell testing in both sexes of 81 milligrams of acetyl salicylic acid a day, Hopkins researchers found aspirin therapy prevents the clumping together of these clot-forming cells, called platelets. Clots in blood vessels of the heart and brain can cause heart attacks and strokes.

The study findings challenge the conclusions from several other recent studies, including the federal Women’s Health Study, which showed low-dose aspirin had no effect in preventing heart attacks in women, even though it worked in men. Previous results, the researchers say, were not likely caused by the failure of aspirin to prevent platelets from clumping together and forming blood clots in women.

“Women are clearly benefiting from taking aspirin and should continue to take it to improve their cardiovascular health,” says study senior investigator Diane Becker, M.P.H., Sc.D., a professor at The Johns Hopkins University School of Medicine and Bloomberg School of Public Health. “Aspirin has been proven by all previous studies to lower the risk of stroke and, as our latest findings show, it also reduces platelet aggregation that can lead to potentially fatal clots in blood vessels.”

“Our results show that aspirin does what it is supposed to do in both men and women,” says platelet biologist and study co-author Nauder Faraday, M.D., an associate professor at Hopkins. “But women started at a higher baseline level of platelet aggregation and remained slightly higher even after taking aspirin. So, it remains unclear if the residual differences in platelet function impact the drug’s overall beneficial effects, and if the doses used in earlier studies were sufficient to decisively prevent heart attacks in women.

Results in both men and women showed that aspirin, taken daily for a two-week period, works by inhibiting key biological pathways that lead to platelet clumping.

Using an electrical measure of how well platelets stick together, researches found that in aspirin-treated men, clumping decreased by 15.1 ohms. The decrease was statistically the same in aspirin-treated women, at 17.3 ohms. In this test, an ohm is the measure of electrical resistance caused by platelets as they impede the flow of electricity in a wire probe inside a test tube filled with blood.

Moreover, platelet aggregation was largely suppressed in at least three other key pathways related to their function when platelets were stimulated with substances that normally trigger clot formation. Each of these tests involved mixing whole blood, or platelet-rich plasma, from aspirin-treated men and women with various concentrations of each of the main chemical compounds involved in the pathways -- collagen, adenosine diphosphate, and epinephrine - to see how platelets responded.

Further analysis of results highlighted mainly two factors, platelet reactivity levels before therapy starts and gender, as having played a significant role in predicting the effects of aspirin therapy on platelet clumping. Other factors, such as age, race or known risk factors for heart disease, including smoking, obesity and high blood pressure, were not found to be good predictors of aspirin’s beneficial effects.

More than 500 men and 700 women participated in the study, called the Genetic Study of Aspirin Responsiveness (GeneSTAR). Blood testing was conducted both before and after treatment. At the start of the experiment, laboratory tests of blood platelets in women were found four times more likely than in men to aggregate when exposed to arachidonic acid, a clot-inducing chemical in the pathway that is most suppressed by aspirin.

While taking aspirin, participants maintained a strict and consistent dietary and exercise regimen, with no smoking or consumption of foods that by themselves affect platelet activity, such as caffeine, chocolate, wine or grapefruit juice. Physical examinations and pill counts were conducted to ensure that all participants adhered to the study protocol. Because aspirin reaches its maximal effect in the body at five days, the researchers say a longer study testing period was not required to determine the drug’s effects on platelet function.