The research, being presented today at the American Stroke Association's conference in Phoenix, finds that people at risk for a stroke who take baby (also known as low-dose) aspirin were less likely to enjoy its blood-thinning effects than those taking an adult-size pill. The same was true of those who took coated aspirin compared to the uncoated kind.
"If this line of research continues to pan out, it could have significant public health implications because it could lead to the concept of aspirin as a dose-adjusted medication," says Dr. Mark Alberts, lead author of the study and director of the stroke program at Northwestern Memorial Hospital in Chicago.
Instead of a one-size-fits-all approach, he says, "we would measure the effect of the dose and consider whether the patient needs something different. There clearly appear to be some differences in how patients respond."
Right now, doctors consider that anywhere from 50 milligrams to 325 milligrams of aspirin may help prevent against ischemic stroke, says Dr. Marshall Keilson, associate director of neurology at Maimonides Medical Center in Brooklyn, N.Y. Ischemic strokes are the most common type of stroke and occur when a narrowed artery or blood clot impedes blood flow in an artery supplying the brain. Aspirin thins the blood by preventing platelets from aggregating. This, in turn, prevents clots from forming.
Within that range, Keilson says, "we've been using low doses because we feel it may reduce potential complications like gastrointestinal upset and bleeding."
And many patients taking aspirin are still having strokes and, for that matter, heart attacks (aspirin is also thought to be able to prevent some heart attacks). "They are implying that the low doses that are commonly being used -- around 81 milligrams -- may not be adequate for stroke prevention," Keilson says.
In this study, Alberts and his co-authors used a machine called a PFA 100 to measure how well different doses and formulations of aspirin were able to thin blood in patients who had cerebrovascular disease. Patients were divided into groups receiving either a high (325 mg, an adult pill) or low (81 mg, or baby pill) dose of aspirin and further separated into coated or uncoated aspirin.
"We found that over half of the patients taking 81 milligrams a day did not have an antiplatelet effect, which was significant," Alberts says. (The figure was actually 56 percent.) "Out of the patients who were taking coated aspirin, over 60 percent did not have an antiplatelet effect compared to the people taking 325 milligrams of uncoated aspirin."
In other words, higher doses and uncoated formulations seemed to work better.
Aspirin also appeared to be less effective in thinning blood in older patients and more effective in women than in men.
Of course, there's a long way to go before these early results become routinely recommended.
The study measured only thinning of the blood, not whether a person had another stroke. "We don't know which of these PFA results correlate with clinical outcomes," Alberts says. "We want to see if this is predictive of clinical results, which is the holy grail of research."
Keilson agrees. "Sometimes things that are done in a laboratory don't carry over in people," he says. "We need a large study, a multicentered trial with thousands of patients."
If the findings are borne out, the PFA 100, a device the size of a coffee maker, could be a quick, easy and inexpensive way to determine how well aspirin is doing its job in a particular individual. According to Alberts, each test takes about three minutes and costs only $10 to $15.
The findings may also one day have implications for preventing heart attacks.
"There is a strong overlap of risk factors," says Dr. Dan Fisher, a cardiologist with New York University Medical Center in New York City. "We're still trying to find the right time to use aspirin. The guidelines are not perfect. We don't have clear answers."
One thing is clear: There is a need for more precise guidelines.
"Somewhere between 50 and 325 milligrams is effective. That's a big range. It allows for different types of treatment in different parts of the country," Keilson says. "There should be a uniformity of how you take care of people."
For more on the different types of stroke, visit the American Stroke Association. The association also has information on current guidelines for the use of aspirin in preventing strokes and heart attacks.