Warfarin (Coumadin)

Action

• inhibits Vit K dependent production of clotting factors
•  mainly inhibits prothrombin (II) and VII but also IX and X
•  VII (t/2=4-6hrs), IX (t/2=18-24hrs), X (t/2=18-24hrs), II (t/2=18-24)
•  proteins C (t/2=6 hrs) and S (early);
•  hence PT increases in 48 hrs 2nd to decr of VII but many days of therapy needed for therapuetic levels of other the factors;

Dose

• usually given within 3 days of heparin therapy;
• loading Dose: 5-15 PO/IM/IV qd for 1-5 days;
• maintenance: 2-15mgPO/IM/IV;
• supplied: 1.0, 2, 2.5, 5, 7.5, 10mg;

Labs

• titrate dose keeping PT = 1.2-1.5 x normal;
• initially check PT PT & PTT qd; PT incr. in less than 48 hrs;
• expect therapuetic PT in 5-6 days; Then check PT q1-2 weeks;

Precautions

• warfarin induced skin necrosis:
  • be careful with patients w/ elevated platlet levels;
• warfarin interactions and resistance:
  • avoid ASA and IM injections;
  • acetaminophren may interfere with hepatic metabolism of warfarin (and thus may increase INR);
• note that Vit K 10-50mg SC/IM or for rapid (but expensive) correction may use FFP  (2 units at a time) / check PT;
• for Prolonged Reversal of Anticoag: Vit K 10 mg PO/SC/IV for 3 days

References »

• New Anticoagulants — The Path from Discovery to Clinical Practice
• The use of novel oral anticoagulants for thromboprophylaxis after elective major orthopedic surgery.
• Prevention of venous thromboembolic disease after total hip and knee arthroplasty.