The mechanism of aspirin's antiplatelet activity is acetylation of platelet cyclo-oxygenase, which causes irreversible inhibition of platelet-dependent thromboxane formation.
Features of aspirin resistance:
(1) presence of thrombotic event while on aspirin therapy
(2) no evidence of decreased platelet function in laboratory tests
(3) exclusion of alternative explanations
Laboratory tests:
(1) platelet aggregation studies (optical aggregometer)
Exclusions:
(1) noncompliance
(2) inadequate dose
(3) hypercoagulable state
(4) nonthrombotic vascular disease
(5) drug interaction interfering with aspirin effect
If a patient is suspected of having aspirin resistance, a number of therapeutic options are available:
(1) clopidogrel
(2) dipyridamole plus aspirin
(3) warfarin plus aspirin
