Table 2: Platelet function tests and commentary [18, 21].

(1) Thromboxane A2 synthesisMeasurement of metabolites such as serum thromboxane B2 or urinary 11-dehydro-thromboxane B2, direct metabolites of COX-1, a specific mechanistic target of aspirin may be made. These tests are limited by a nonlinear relationship between platelet COX-1 activity and thromboxane A2 activity, and extra-platelet sources of thromboxane A2 synthesis. Furthermore, urinary excretion of 11-dehydro-thromboxane B2 must be normalized according to urinary function (e.g., creatinine concentration).

(2) Aspirin response according to thromboxane dependent assays light transmittance aggregometry (LTA), impedance aggregometry (IA), platelet function analyser (PFA-100), and VerifyNowLTA measures light transmission through a platelet suspension exposed to a platelet agonist such as ADP, but the agonists may activate pathways less dependent on COX-1. IA measures electrical impedance after exposure to whole blood suspension by a platelet agonist. There may be poor reproducibility, variation of response by age, race, sex, hematocrit, and concentration of the agonist. Like IA, LTA may be associated with poor reproducibility. PFA-100, an in vitro recorder, includes a membrane with an aperture coated with collagen plus an agonist (e.g., epinephrine, ADP). As platelets form aggregates, the aperture occludes, and flow factors may affect test results (e.g., nonsteroidal anti-inflammatory drugs, clopidogrel, GP IIb/IIIa expression on the platelet surface, von Willebrand factor, platelet count, hematocrit, and diurnal variation (lower closing times in the morning)). VerifyNow measures platelet function by light transmission through a suspension of lyophilized fibrinogen-coated beads and an agonist such as arachidonic acid.
Clopidogrel Response. Platelet function measured by LTA using the agonist, ADP, before and after treatment, is the main standard test to assess clopidogrel. Point-of-care assays such as VerifyNow, Thromboelastography (discussed below), and PFA-P2Y may be employed. These tests all have limitations that are discussed elsewhere (see [21]).

(3) ThromboelastographyMeasures the contribution of ADP-induced aggregation to tensile strength of platelet-fibrin clot and requires further validation studies as does the PFA P2Y test that measures clopidogrel response.

(4) Degree of phosphorylation of VASPClopidogrel irreversibly blocks the ADP receptor P2Y12 and activates a cAMP-dependent protein kinase a that inhibits VASP, vasodilator-stimulated phosphorylation. VASP is an inducer of platelet aggregation via GP IIb/IIIa. The degree of phosphorylation of VASP to an antiplatelet agent may be determined by flow cytometry, but there may be limited sensitivity.