Aspirin resistance: is this term meaningful?

PURPOSE OF REVIEW: To review data for and against the existence of 'aspirin resistance', a term coined to indicate aspirin-treated patients having ex-vivo tests of platelet activation insensitive to aspirin treatment and recurrence of cardiovascular disease. RECENT FINDINGS: 'Aspirin resistance' defined by ex-vivo tests of platelet activation yielded values ranging from 21 to 78%, indicating that such tests do not provide a useful measurement. In long-term aspirin-treated patients, studies demonstrated small but functionally relevant platelet thromboxane A2 formation that was responsible for an enhanced platelet activation in response to platelet agonist. These studies, however, did not fully exclude that aspirin compliance may be implicated in such phenomena. Two trials performed in patients with coronary artery disease demonstrated that laboratory evidence of aspirin resistance was no longer detectable when aspirin compliance was accurately monitored. SUMMARY: Given the multifactorial nature of atherothrombosis, recurrence of cardiovascular events in aspirin-treated patients does not necessarily suggest 'drug failure'. A cause-effect relationship between platelet insensitivity to aspirin and cardiovascular recurrence has not been defined overall because aspirin compliance has been scarcely considered. Until this information is taken into account, the existence of 'clinical resistance' to aspirin should be reconsidered.