Principle and kinetics of the serum TXB₂ ex vivo assay. (A) The chain of enzymatic reactions triggered by whole blood clotting in vitro at 37°C. Thrombin generated during blood clotting activates platelet phospholipases A₂ (PLA₂), which releases AA from membrane phospholipids. AA is the substrate of the sequential action of COX-1 and TX synthase, resulting in TXA₂ generation. Because of chemical instability of its oxane ring, TXA₂ is rapidly hydrolyzed to the chemically stable, biologically inactive hydration product, TXB₂, which can be measured in serum with high sensitivity and specificity. (B) The kinetics of TXB₂ production during whole blood clotting at 37°C and based on data from Patrono et al.⁶¹