Abstract 5006: Aspirin Non-Response as Determined by Multiple Platelet Function Assays
Background. While low dose aspirin has become the standard of care for the prevention of cardiovascular events there is evidence that enteric aspirin may not be as effective as plain aspirin.
Methods. We screened 138 patients with cardiovascular disease who were taking aspirin and measured their aspirin response with three platelet function assays: Light Transmission aggregometry (LTA) with arachidonic acid, the VerifyNow™-Aspirin and Thromboxane B2 (TxB2) ELISA. Resistance was defined as >20% aggregation by LTA, >550 ARU in the VerifyNow™ and >10 ng/ml by TxB2 ELISA. Patients aspirin preparations were altered and dosages increased to a maximium of 300mg plain aspirin if patients showed lack of response to their aspirin by any one assay.
Results. 24% of patients were non-responders in at least one assay. There was 13% agreement between LTA and TxB2, 9% between TxB2 and VerifyNow™ and 19% between VerifyNow™ and LTA. Only 9% of patients defined as resistant by one assay were resistant to all three. Upon repeat measurement, with compliance encouraged (through education) and assured (directly observed) only 6.5% of patients were still non-responders in at least one assay. Compliant non-responding patients were heavier than the responding population (98.5kg versus 77.9kg). Only 2.7% (105kg) of the cohort remained non-responders after swtiching their aspirin preparations from enteric-coated to plain. Increasing aspirin dosage to 150mg plain aspirin resulted in 0.4% non-response.
Conclusions. These data show that whilst aspirin non-response is a problem, there is significant inter-assay agreement making it difficult to define. However, over 2/3 (69.6%) of non-responding patients became responders after encouraging compliance. The remaining patients were heavier patients and switching to plain aspirin which has a higher bioavailability improved their response to aspirin. Only 1 patient could be detected who did not respond to aspirin suggesting that aspirin resistance is very rare and most cases are due to poor compliance or underdosing in heavier patients exacerbated by the lower bioavailability of enteric-coated aspirin. While there was poor agreement between assays they were effective at identifying non-compliant patients.