VANCOUVER, B.C.–Results of a small prospective study suggest that almost half of patients hospitalized for treatment of ischemic stroke or transient ischemic attack (TIA) are “aspirin resistant.”
But some stroke experts are not convinced, citing a lack of evidence that the results of the platelet function analyzer (PFA-100) assay used in the study are a true measure of antiplatelet function.
Of 59 patients tested, 47% had aspirin resistance, which was defined as a clotting time of 171 seconds or less, Dr. Mark J. Alberts reported at the Fifth World Stroke Congress, sponsored by the International Stroke Society.
In an earlier study of outpatients, Dr. Alberts, professor of neurology at Northwestern University, Chicago, reported that 37% of such patients were aspirin resistant. That study, first reported at the American Stroke Association meeting in 2003, was recently published (Stroke 35[1]:175–78, 2004).
In the later study, the 59 patients had a mean age of 64 years and had been taking aspirin for at least 3 days prior to an acute event (stroke or TIA). PFA-100 testing was done at the time of hospital diagnosis, before any loading dose of antiplatelet therapy was administered. The PFA-100 device measures the clotting time of a blood sample that is pumped through a collagen-coated membrane, a test design that mimics the behavior of circulating blood. The device, about the size of a drip coffee maker, costs about $15,000 and produces results in about 5 minutes at a cost of $15–$20 per assay, he said.
Overall, 63% of patients were taking 325 mg/day of aspirin and 37% were using baby aspirin (81 mg). Aspirin resistance was seen in 73% of patients taking low-dose aspirin and in 32% of those taking high-dose aspirin, a significant difference. Patients using enteric-coated aspirin also were more likely than others to show resistance.
The results suggest that “dose-adjusted therapy is where the field is headed. … 'One size fits all' therapy doesn't work for aspirin,” Dr. Alberts said.
Dr. Ralph L. Sacco, director of the stroke and critical care division at Columbia University, New York, said in an interview that Dr. Alberts' findings were “thought-provoking, but I don't know how an abnormal finding on PFA correlates with clinical outcomes.”