Introduction: Thromboxane A2 (TXA2) produced from arachidonic acid by the consecutive action of cyclooxygenase 1 (COX-1) and thromboxane synthase during platelet activation plays an important role in the process of thrombosis. It however has a short half-life and is soon hydrolyzed to form thromboxane B2 (TXB2), an inactive metabolite of thromboxane A2. Aspirin blocks TXA2 formation by irreversibly inhibiting COX-1. During blood coagulation platelet activation is induced by the generated thrombin, which also induces TXA2 formation. The measurement of TXB2 in the serum of in vitro clotted blood is used to monitor the effect of aspirin. However, this method has not been properly evaluated in various clinical conditions.
Aims: As anticoagulation impairs thrombin generation, and the effect of aspirin is often tested in anticoagulated patients, we investigated how anticoagulation by heparin influences TXB2 production during in vitro blood clotting.
Methods: Peripheral blood was drawn from the antecubital vein of 9 individuals. Blood samples were supplemented with various concentrations of nonfractionated heparin to cover heparin concentration in the therapeutic range (0-0.6 U/mL). Coagulation was induced by CaCl2. After one hour incubation at 37 °C the samples were centrifuged at (1200 g, 10 min) and the serum was used for TXB2 determination by competitive immunoassay (Assay Designs, Ann Arbor, MI). The effect of heparin on recalcification clotting time (RCT) was also monitored.
Results: In the absence of heparin the TXB2 generation during the coagulation of whole blood varied in a range of 86- 210 ng TXB2/million platelets (mean±SD: 128±43). 0.25-0.35 U/mL and 0.45-0.55 U/mL plasma heparin concentration decreased the TXB2 production to 71±58, range 16-186 ng TXB2/million platelets and to 18±10, range: 7-37 ng TXB2/million platelets, respectively. The decrease in TXB2 production correlated with the prolongation of RCT, i.e, with the decrease of thrombin generation.
Conclusion: As anticoagulation dose dependently decreases TXB2 production in the clotting blood, TXB2 measurement is not a suitable technique for monitoring aspirin therapy in anticoagulated patients.