Platelet Function Tests: Why They Fail to Guide Personalized Antithrombotic Medication

General Considerations D espite the essential role of platelets in arterial thrombogenesis, personalized antiplatelet therapy based on platelet function tests (PFTs) did not improve clinical outcome after coronary revascularization and did not predict recurrent ischemic or bleeding events in individual patients. To understand why these point-of-care PFTs have yielded so little clinical benefit, we scrutinized their mechanism of thrombus assessment for its relevance to the pathomechanism of arterial thrombogenesis. Fundamental differences exist between thrombus generation at high shear in native blood in vivo and agonist-induced platelet aggregation at low shear in anticoagulated blood ex vivo. Most PFTs in current use reproduce the second scenario. These tests, which are variants of the classical platelet aggregometer, are based on the assumption that the secretion of agonists from activated platelets is the major determinant of thrombus growth. Accordingly, various soluble agonists are used (adenosine diphosphate [ADP], thromboxane A2 [TXA2], or a-thrombin) to activate platelets in citrateanticoagulated whole blood at very low shear, and the formation of small platelet aggregates (attached to fibrinogen-coated beads to obtain better optical signals) are measured. Under physiological conditions, at arterial shear rates ( 420 s ), platelet aggregation occurs only in response to activation by agonists; however, in vivo, at sites of turbulent flow near the site of a severe stenosis and thus high shear (above 10 000 s ), long-lasting adhesion and aggregation occur without any requirement for platelet activation. Consequently, the main soluble agonists are not involved in the initial shear-induced platelet aggregation but rather contribute to stabilization of the unstable platelet aggregate. There are also essential differences between platelet aggregation in citrated blood and native blood. In citrated blood, in response to activation of platelets by ADP, collagen, or arachidonic acid, soluble agonists are released from platelet storage granules or generated by platelets (release reaction), which substantially enhances the initial platelet reaction (secondary aggregation). In contrast, in native blood, whether in vivo or in vitro, only activation by a-thrombin binding to platelet surface glycoprotein Iba receptors can induce platelet adhesion, dense granule secretion, and aggregation. Under pathological conditions, thrombin is essential not only for initiation and propagation of the initial platelet aggregation but importantly for stabilization of the platelet thrombus through enzymatic and structural effects. By releasing plasminogen activator inhibitor 1, the main inhibitor of the fibrinolytic system, from the storage granules of platelets and activating thrombin-activatable fibrinolysis inhibitor, thrombin confers resistance on the arterial thrombus against endogenous fibrinolysis. Thrombin also anchors unstable platelet aggregates, together with fibrin, to the site of vascular injury, imparting structural stability to the thrombus, thereby preventing downstream embolization due to the effects of arterial flow (Figure 1).