Lower Antiplatelet Effect of Aspirin in Essential Thrombocythemia than in Coronary Artery Disease.

Background  Patients with essential thrombocythemia (ET) and coronary artery disease (CAD) have increased risk of thromboembolic complications. In addition, a reduced antiplatelet effect of aspirin has been demonstrated in both patient groups. As ET is a platelet disorder, platelets may be more important for the thromboembolic risk in ET than in CAD. We aimed to investigate the antiplatelet effect of aspirin and platelet turnover in ET versus CAD patients. Methods  We included 48 ET patients and an age-matched group of 48 CAD patients. The effect of aspirin was evaluated by thromboxane B 2 (TXB 2 ) levels and platelet aggregation. Platelet turnover was assessed by immature platelet count (IPC) and immature platelet fraction (IPF). Results  ET patients had reduced effect of aspirin compared with CAD patients, demonstrated by significantly higher TXB 2 levels (median of differences = 22.3 ng/mL, p  < 0.0001) and platelet aggregation (median of differences = 131.0 AU*min, p  = 0.0003). Furthermore, ET patients had significantly higher IPC ( p  < 0.0001) and IPF ( p  = 0.0004) than CAD patients. Conclusion  ET patients have lower 24-hour antiplatelet effect of aspirin than CAD patients. This may be explained by an increased platelet production and turnover counteracting the antiplatelet effect of aspirin. These findings strengthen the rationale for exploring novel antiplatelet regimens in ET patients to reduce the risk of cardiovascular events.