Aspirin resistance is rare

Pharmacological resistance to aspirin is rare. Researchers at the University of Pennsylvania in Philadelphia said they could not identify one case of true drug resistance, according to the study published online Dec. 4 in the journal Circulation.

"Low-dose aspirin reduces the secondary incidence of myocardial infarction and stroke. Drug resistance to aspirin might result in treatment failure," the authors wrote. "Despite this concern, no clear definition of 'aspirin resistance' has emerged and estimates of its incidence have varied remarkably."

The investigators observed the response of 400 volunteers to a single oral 325-mg dose of immediate-release or enteric-coated aspirin. The researchers noted that individuals who appeared aspirin resistant eventually responded and that enteric-coated aspirin may complicate administration.

The authors examined individuals who appeared aspirin resistant on one occasion and who underwent repeat testing. If they appeared resistant again, they were exposed to low-dose (81 mg) enteric-coated aspirin and clopidogrel (75 mg) for 1 week each.

The authors noted that nearly half of the patients appeared resistant to a single dose of 325-mg enteric-coated aspirin due to variable absorption. There were no patients resistant to immediate-release aspirin. All of the patients responded after repeated exposure, extension of the post-dosing interval, or addition of aspirin to their platelets ex vivo, the authors note in their abstract.

Although the authors could not document a true case of aspirin resistance, they noted that "pseudoresistance, reflecting delayed and reduced drug absorption, complicates enteric coated but not immediate-release aspirin administration."