Antithrombotic activity and its relationship to in vitro anticoagulant effects.

Antithrombotic activities of low molecular weight heparins (LMWHs) in a venous stasis thrombosis model, when compared to unfractionated heparin (UFH), correlated better with anti-IIa activities and inhibition of thrombin generation than with anti-Xa activities. The relative antithrombotic potencies of LMWHs were closer to inhibitory activity in thrombin generation with platelet-poor plasma than platelet-rich plasma. Comparing activities in intrinsic and extrinsic systems, higher concentrations of heparins were required in vitro to inhibit intrinsic thrombin generation than extrinsic thrombin production; however, higher doses were required to prevent in vivo thrombotic events initiated via the extrinsic pathway.