Arachidonic acid (AA) stimulation of platelet aggregation depends directly on cyclooxygenase (COX)-1 activity. In vivo production of thromboxane A₂ (TxA₂) is assessed by measurement of stable metabolites via enzyme-linked immunoassays. Adenosine diphosphate (ADP)- and collagen-stimulated aggregation are COX-1–nonspecific methods. Aggregation occurs through COX-1–independent and –dependent pathways after stimulation by the latter agonists. ASA = acetylsalicylic acid (aspirin); GP = glycoprotein; LTA = light transmittance aggregometry; PFA = platelet function analyzer; PLA₂ = phospholipase A₂; PRP = platelet-rich plasma; TEG = thrombelastography.

Adenosine diphosphate (ADP) stimulates 2 distinct receptors (P2Y₁ and P2Y₁₂) that are linked to specific signaling pathways (dashed arrows). Response can be measured by: (A) receptor reactivity; intracellular signaling downstream from the P2Y₁₂ receptor is measured by flow cytometry that assesses phosphorylation of vasodilator-stimulated phosphoprotein (VASP) with monoclonal antibodies (Y); the P2Y₁₂ is coupled by a G_i protein to adenylyl cyclase; PKA = protein kinase A; (B) activation-dependent receptor expression (active glycoprotein [GP] IIb/IIIa and P-selectin) identified by monoclonal antibodies (Y) with flow cytometry; (C) aggregation determined by light transmittance aggregometry (LTA), aggregation of platelets with fibrinogen-coated beads (VerifyNow), or measuring the contribution of platelet aggregation to total platelet–fibrin clot strength by thrombelastography.

Intersubject variation in platelet aggregation in response to various agonists in citrate-anticoagulated samples. ADP = adenosine diphosphate; CRP = collagen-related peptide. Reprinted from Yee et al. (38), with permission.

Presentation of platelet function data as a histogram of the absolute change in aggregation (A) in response to 5 µmol/l adenosine diphosphate (ADP) stimulation at 2 h after treatment with 300-mg clopidogrel loading dose. The A is defined as baseline aggregation (%) minus the post-stent aggregation (%) and is divided into deciles. Nonresponsiveness or resistance is defined as A 10%. Nonresponsiveness is present in those patients below the solid arrow. Heightened post-treatment reactivity is present in those patients below the dashed arrow. The curve represents a normal distribution of data (Statistica software, Tulsa, Oklahoma). Adapted from Gurbel et al. (50), with permission.

Platelets mediate an array of in vivo processes leading to adverse events. Initial activation is stimulated by key primary agonists (collagen, von Willebrand factor [vWF], thrombin). The activated platelet surface is the site of coagulation-factor generation, leading to amplification of thrombin generation. Release of key secondary agonists (adenosine diphosphate [ADP], TxA₂) upon activation amplifies aggregation. PLT = platelet; TF = tissue factor; WBC = white blood cell; other abbreviations as in Figure 1.

Relation between creatine kinase-MB (CKMB) release and mean platelet aggregation in response to 5 µmol/l adenosine diphosphate measured serially over 18 to 24 h in patients undergoing stenting in the CLEAR PLATELETS study. Adapted from Gurbel et al. (21) with permission. NL = normal limit; ULN = upper limit of normal.

(A) Distributions measured in patients with and without stent thrombosis. Data from Gurbel et al. (26). (B) Distributions measured in patients with and without ischemic events within 6 months of elective coronary-artery stenting; after percutaneous coronary intervention (PCI) measurement. Data from Gurbel et al. (20). (C) Distributions measured in patients with and without ischemic events within 1 year treated with clopidogrel therapy before elective coronary artery stenting; before PCI measurement. Data from Bliden et al. (23). ADP = adenosine diphosphate; ST = stent thrombosis.