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The page below is a sample from the LabCE course Antiplatelet and Anticoagulant Pharmacology for the Laboratory Professional. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Warfarin

  • Warfarin is a small molecule that is able to be absorbed by the gastrointestinal system. It is available in a pill form.
  • Mechanism of action: Warfarin prevents vitamin K from modifying factors II, VII, IX, and X in the liver. Without this modification from the liver, those factors cannot function in the coagulation cascade appropriately. This action by warfarin results in anticoagulation. Warfarin cannot change factors that have already been made by the liver, so its impact is not as immediate as heparin.
  • Use: Warfarin is used in the chronic management of excessive coagulation (hypercoagulable state). It is common to see a "heparin bridge," during which time heparin or LMW heparin is used for a short period while the patient also begins their warfarin. This allows the warfarin to obtain a therapeutic blood level. The goal is to achieve an INR of 2-3 in hypercoagulable states or 2.5-3.5 in patients with mechanical heart valves. As soon as the target INR is obtained, heparin is stopped.
  • Laboratory measurement: The PT/INR assay monitors warfarin. As with the aPTT, the PT assay is measured in seconds. Clinicians use the calculated International Normalized Ration (INR) value to determine the effectiveness of a patient's warfarin, however. The INR result accounts for differences in reagents from laboratory to laboratory, allowing a physician to understand the result no matter where a patient's PT is tested.
  • Toxicity: Anticoagulants can cause excessive bleeding and warfarin is no different. If a patient has a supratherapeutic (above goal) INR, vitamin K is given by either orally or via injection. The vitamin K injection can overwhelm the vitamin K inhibiting properties of warfarin, but this is a slow process. If rapid measures are needed, fresh frozen plasma (FFP) is used as a supply of coagulation factors. In 2016, a new treatment for rapid warfarin reversal became available- prothrombin complex concentrate (PCC). Note that warfarin cannot be used in pregnancy due to teratogenic effects.