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Initiation Of Therapy Days 1-4

  • Warfarin is generally started on day 1 or 2 of heparin or low molecular weight (LMW) heparin therapy. Heparin or LMW heparin administration should overlap with warfarin for a minimum of 6 days and until INR is within the desired therapeutic range on 2 consecutive days at least 24 hours apart when initiating warfarin therapy. In general, warfarin therapy should be initiated with consultation from hematology unless the patient is in a critical care unit or on the cardiology or cardiothoracic surgery service.

  • Target INR (International Normalized Ratio):

    • 2.5 to 3.5 for patients with mechanical/prosthetic mitral valves or recurrent thrombotic events with a therapeutic INR.

    • 2.0 to 3.0 for all other patients including patients with mechanical aortic valves.

  • Obtain blood for baseline INR/PT, aPTT.

  • Calculate initial (day 1) warfarin dose based on weight, co-morbidities and baseline INR.

    • Patient with a baseline INR < 1.2 and no liver or Fontan co-morbidities or hemorrhagic risk:

      • 0.2 mg/kg PO as a single dose.

      • Maximum 10 mg.

    • Patient with a baseline INR ≥ 1.2 - Consult Hematology.

    • Patient with liver dysfunction, Fontan procedure, presence of other hemorrhagic risk (hemodialysis):

      • 0.1 mg/kg PO as a single dose.

      • Maximum dose 5 mg.

  • Obtain daily INR during initiation protocol.

  • Calculate subsequent (days 2-4 only) warfarin initiation doses based on the INR response (see Table 3).

Table 3. Adjusting Warfarin Dose for Days 2 to 4 ONLY


Warfarin Adjustment


repeat initial loading dose


50% of initial loading dose


25% of initial loading dose

> 3.5

hold until INR < 3.5; restart at 50% of previous dose


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Horton, JD, et al. Warfarin Therapy: Evolving Strategies in Anticoagulation. American Family Physician; 1999 Feb; 59(3):635-646.

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These guidelines do not establish a standard of care to be followed in every case. It is recognized that each case is different and those individuals involved in providing health care are expected to use their judgment in determining what is in the best interests of the patient based on the circumstances existing at the time. It is impossible to anticipate all possible situations that may exist and to prepare guidelines for each. Accordingly these guidelines should guide care with the understanding that departures from them may be required at times.