Comparison of Three Different Platelet Function Assays to Determine Aspirin Sensitivity.

In large studies of patients with cardiac disease, it was observed that up to 9% are ASA resistant (ASAR) and 23% are ASA semi responders using the platelet function analyzer (PFA-100), which measures whole blood platelet adhesion and aggregation under high shear. The prevalence of ASAR is of great clinical importance. With an estimated 20 million patients in the USA taking ASA for prevention of atherosclerotic events, even a 10% incidence equates to more than two million patients receiving inadequate anti-platelet therapy. Given the widespread use of ASA, more reliable and rapid methods to measure aspirin sensitivity are needed. Importantly, future large scale studies to determine the effect of monitoring ASA sensitivity to optimize therapy are compromised due to current lack of uniformity in assessing ASAR from center to center. The classical method of platelet aggregometry is labor-intensive and not readily adaptable to the clinical setting. Moreover, platelet aggregometry measures platelet responses under low shear, which does not simulate the high shear conditions expected to be involved in arterial thrombosis. In contrast, the PFA-100 does measure thrombus formation under high shear. The new TEG platelet coagulation assay uses whole blood, includes high shear, and is a point of care method. We compared these three techniques to assess aspirin effects on platelet function and clot formation: PFA-100 with collagen-epinephrine cartridges, TEG platelet coagulation assay, and standard optical aggregation in platelet rich plasma using arachidonic acid (AA) and adenosine diphosphate (ADP) as agonists. RESULTS: We found significant differences between the PFA-100, TEG platelet function, and standard optical aggregation. Twelve normal individuals previously defined as ASA sensitive or ASAR, using the PFA-100, were studied at baseline and following three days of oral aspirin at 81mg/day. We found that all four patients determined to be ASAR by PFA-100 were found to be sensitive in TEG and aggregometer assays when using AA as the agonist. Furthermore, some participants showed a gain of platelet function following ASA when studied on the TEG and aggregometer using ADP as the agonist. Currently, it is unclear which response to ASA treatment is most important to predict cardiovascular complications in normal individuals or patients placed on aspirin. Gum et al, showed that increased urinary secrection of thromboxane metabolites, suggesting ASA insensitivity, is associated with a higher incidence of cardiovascular events. Unfortunately, this test may simply indicate poor patient compliance rather than true ASAR, so a clear demonstration of platelet sensitivity will also be necessary. Our data confirms the need for a collaborative trial comparing different platelet assays to assess true ASA sensitivity together with concurrent measurements of urinary thromboxane metabolite levels. Knowing which assay best links aspirin sensitivity to disease outcome will allow physicians to better manage normal individuals and patients at risk for significant cardiovascular disease.