Warfarin, heparin, and low-molecular-weight heparin (LMWH) have been used for decades as anticoagulants. Before 2010, the vitamin K antagonist warfarin (Coumadin) was the only class of oral anticoagulants available.
The extrinsic pathway functions outside the bloodstream, initiated by tissue factors. Warfarin mainly influences this pathway because it inhibits hepatic synthesis of factor VII, the most essential factor in the extrinsic pathway. For this reason, the anticoagulant activity of warfarin must be monitored frequently using the prothrombin time (PT), which is now standardized as the international normalized ratio (INR).
Heparin works as a thrombin inhibitor by binding to antithrombin. Heparin binding to antithrombin potentiates the effect of antithrombin, which neutralizes factor IIa (thrombin).
The activated partial thromboplastin time (aPTT) monitors Heparin in the laboratory.
