Crucial roles of red blood cells and platelets in whole blood thrombin generation

: Red blood cells (RBC) and platelets contribute to the coagulation capacity in bleeding and thrombotic disorders. The thrombin generation (TG) process is considered to reflect the interactions between plasma coagulation and the various blood cells. Using a new high-throughput method capturing the complete TG curve, we were able to compare TG in whole-blood and in autologous platelet-rich and platelet-poor plasma to redefine the blood cell contributions to the clotting process. We report a faster and initially higher generation of thrombin and a shorter coagulation time in whole blood than in PRP upon low concentrations of coagulant triggers, including tissue factor, Russell's viper venom factor X factor Xa, factor XIa and thrombin. The TG accelerated with increased hematocrit, and delayed after prior treatment of RBC with phosphatidylserine-blocking annexin A5. RBC treatment with ionomycin increased phosphatidylserine exposure, confirmed by flow cytometry, and enhanced the TG process. In reconstituted blood samples, annexin A5-blocked RBC allowed an enhanced glycoprotein VI-induced platelet procoagulant activity. In patients with anemia or erythrocytosis, cluster analysis revealed high or low whole-blood TG profiles in specific cases of anemia. All TG profiles lowered upon annexin A5 addition, and were thus determined by the extent of phosphatidylserine exposure of blood cells. Profiles for patients with polycythemia vera undergoing treatment were similar to control subjects. We concluded that RBC and platelets, in a phosphatidylserine-dependent way, contribute to the TG process. Determination of the whole-blood hypo-or hyper-coagulant activity may help to characterize a bleeding or thrombosis risk.