Warfarin

Summary

Warfarin is a medication that is used to prevent blood clots, which is why it is classified as a blood thinner or anticoagulant. Clinically, warfarin is used to prevent the formation of DVTs, pulmonary embolisms, heart attacks, and strokes in patients who are at high risk of developing blood clots. Along with its blood thinning action comes an increased risk for uncontrolled bleeding. To reduce the risk of severe bleeding, patients are monitored using the PT and INR levels. A therapeutic INR level for a patient on warfarin is between 2 and 3. Warfarin is also teratogenic and should not be given to pregnant patients. And finally, patients taking warfarin should avoid eating an excess amount of leafy greens, and other foods high in vitamin K. This is because vitamin K is the antidote for warfarin. In cases where we want warfarin’s effects to be reversed, vitamin K can be given as an injection or through an IV.

Key Points

• Mechanism
  • Anticoagulant
    • Inhibits the liver’s synthesis of vitamin K, which plays a role in decreasing prothrombin and several clotting factors
      • Takes 3-5 days for full effect. Patients may receive heparin concurrently until warfarin has had time to reach a therapeutic level (known as “heparin bridge”)
  • Primarily administered orally
  • Monitor PT/INR
    • INR (international normalized ratio) is a normalized ratio of (patient’s PT)/(international average PT)
    • Therapeutic level of INR is between 2.0-3.0
      • Patients with mechanical heart valves can have INRs as high as 3.5
  • Clinical Use
    • Prevent blood clots
      • Deep vein thrombosis (DVT)
      • Pulmonary embolism (PE)
      • Stroke
  • Side Effects and Adverse Reactions
    • Bleeding
      • Do not take with NSAIDS/aspirin, alcohol, or other agents that may also cause bleeding
      • Monitor for signs and symptoms of bleeding (bruising, tarry stools, sudden drop in blood pressure, petechiae, blood in urine, etc.)
    • Teratogenic
      • Contraindicated in pregnancy
      • Can lead to fetal hemorrhage
  • Antidote
    • Vitamin K
      • Avoid foods high in vitamin K (leafy greens, brussel sprouts, broccoli, green tea), as it may decrease warfarin’s coagulant effect