Anticoagulation Using Warfarin
by Margaret A. Fitzgerald, MS, RN, CS, FNP,
Deanna S. Kania, PharmD, and Ron A. Weathermon, PharmD
Edited by Marty Couret, RN, MSN

Initiating Warfarin Therapy

  1. Starting doses for warfarin are usually between 5 - 7.5 mg.
  2. Prescriptions should have orders to take warfarin in the evening.
  3. Loading doses is no longer recommended.
  4. The condition being treated corresponds to the therapeutic INR range (see Table 1).
  5. The condition also determines the duration of the therapy needed (see Table 2).


Table 1 - Therapeutic INR Range

Baseline INR < 1.5 Dosage
Standard Dose 7.5 mg
Patient weight < 100 lbs. 5 mg
Patient age > 65 years 5 mg
Baseline INR > 20 NO dose - determine cause of elevation

Table 2 - Indications and Length of Warfarin Treatment

Condition

INR

Duration of therapy

Acute venous thrombosis

First episode 2 - 3 3 - 6 months
High risk of recurrence 2 - 3 Indefinitely
In presence of antiphospholipid antibody 3 - 4 Life-long

Prevention of systemic embolis

Tissue heart valves 2 -3 3 months
Valvular heart disease after thrombotic event 2 - 3 Indefinitely
Mechanical heart valve 2.5 - 3.5 Indefinitely
Acute myocardial infarction 2 - 3 As deemed by clinical presentation

Atrial fibrillation

Chronic or intermittent 2 - 3 Lifelong
Cardioversion 2 - 3 3 weeks before and 4 weeks after conversion to sinus rhythm

Reference:
Horton, J., Bushwick, B. (1999) Warfarin therapy: Evolving strategies in anticoagulation. American Family Physician. 59 (3) 635-647.

Laboratory Monitoring Guidelines

  1. INR Monitoring based on stability of the patient
    1. Check INR 2-3 times a week then,
    2. Check INR weekly for two weeks then,
    3. Check INR every 4-6 weeks if INR is stable
      1. Patients are considered stable if they have had two consecutive INRs that have not required dosage change.
      2. Re-check INR no more than 2 weeks after a dose change.
  2. Other Laboratory Monitoring
    1. CBC should be checked every 6-12 months
    2. A urinalysis and hemocult test recommended annually

Reference:
Kania, D. S., Weathermon, R. A.. Anticoagulation monitoring. Notes from clinical skills course 581.

INR Goal 2 - 3

Patient's INR Level

Action

Comment

At desired range

Repeat INR at interval determined by duration of therapeutic INR and underlying condition

 

Repeat INR in

  • 4-6 weeks with stable condition and long-term therapeutic INR
  • At least weekly when underlying condition can impact coagulation state (malignancy, clotting disorder, use of medications that can influence warfarin effect)

INR< 2

  • Increase weekly dose by 5-20%
  • Repeat INR 2-3 times/ week until within desired range

Check for

  • adherence to recommended therapy
  • Use of medications or foods that may interfere with warfarin effect

INR 3-3.5

  • Decrease weekly dose by 5-15%
  • Repeat INR 2-3 times/ week until within desired range

Check for

  • Adherence to recommended therapy
  • Use of medications or foods that may enhance warfarin effect

INR 3.6-4

  • Consider withholding 1 dose, decrease weekly dose by 10-15%
  • Repeat INR 2-3 times/ week until within desired range

Check for

  • Adherence to recommended therapy
  • Use of medications or foods that may enhance warfarin effect

INR >4 without complications and no indication for rapid reversal of anticoagulation effect

  • Consider withholding 1 dose, decrease weekly dose by 10-20%
  • Repeat INR 2-3 times/ week until within desired range

Check for

  • Adherence to recommended therapy
  • Use of medications or foods that may enhance with warfarin effect

INR > 4 and need for rapid reversal of anticoagulant effect

Vitamin K 3 mg SC or slow IV

  • Check INR at 6 and 24 hours
  • May repeat dose

Resource:
Horton, J., Bushwick, B. (1999) Warfarin therapy: Evolving strategies in anticoagulation. American Family Physician. 59 (3) 635-647.

INR 2.5 - 3.5

Patient's INR Level

Action

Comment

At desired range

  • Repeat INR at interval determined by duration of therapeutic INR and underlying condition
  • Repeat INR 2-3 times/ week until within desired range

Repeat INR in

  • 4-6 weeks with stable condition and long-term therapeutic INR
  • At least weekly when underlying condition can impact coagulation state; malignancy, clotting disorder, use of medications that can influence warfarin effect

INR< 2

  • Increase weekly dose by 10-20%
  • Repeat INR 2-3 times/ week until within desired range

Check for

  • Adherence to recommended therapy
  • Use of medications or foods that may interfere with warfarin effect

INR 2- 2.4

  • Increase weekly dose by 5-15%
  • Repeat INR 2-3 times/ week until within desired range

Check for

  • Adherence to recommended therapy
  • Use of medications or foods that may interfere with warfarin effect

INR 3.5-4.6

  • Decrease weekly dose by 5-15%
  • Repeat INR 2-3 times/ week until within desired range

Check for

  • adherence to recommended therapy
  • Use of medications or foods that may enhance with warfarin effect

INR 4.7-5.2

  • Consider withholding 1 dose, decrease weekly dose by 10-20%
  • Repeat INR 2-3 times/ week until within desired range

Check for

  • Adherence to recommended therapy
  • Use of medications or foods that may enhance with warfarin effect

INR >5.2 without complications and no indication for rapid reversal of anticoagulation effect

  • Consider withholding 1-2 doses, decrease weekly dose by 10-20%
  • Repeat INR 2-3 times/ week until within desired range

Check for

  • Adherence to recommended therapy
  • Use of medications or foods that may enhance with warfarin effect

INR > 5.2 and need for rapid reversal of anticoagulant effect

Vitamin K 3 mg SC or slow IV

  • Check INR at 6 and 24 hours
  • May repeat dose

Resource:
Adapted from: Horton, J., Bushwick, B. (1999) Warfarin therapy: Evolving strategies in anticoagulation. American Family Physician. 59 (3) 635-647.

Reversing Over-Anticoagulation With Warfarin

  1. Vitamin K should be administered in cases of extremely high INRs where the patient is not bleeding or at serious risk of having a serious bleed.
  2. Traditional Vitamin K doses were 10 mg IM as a one-time dose.
  3. Low dose Vitamin K (AquaMephyton) is now preferred therapy.
    1. Decreases warfarin resistance
    2. Minimizes patient's length of stay
    3. Able to re-attain therapeutic INR sooner
  4. Any serious, significant bleeding symptoms with associated excessively prolonged INR -- Refer to ER.
INR Level Action
INR < 6 (No bleeding) Hold coumadin
INR 6-10 (No bleeding) Hold coumadin
Vitamin K 0.5 - 3.0 mg sc or 2.5 mg po
Recheck INR in 24 - 48 hours
INR 10-18 (No bleeding) Hold coumadin
Vitamin K 2.5 - 5.0 mg sc or po
Repeat INR in 6 - 24 hours
INR > 18 Vitamin K 10 mg sc
REFER TO ER

For more information

American College of Chest Physicians (ACCP) Guidelines for Oral Anticoagulation Therapy

References:

Horton, J., Bushwick, B. (1999) Warfarin therapy: Evolving strategies in anticoagulation. American Family Physician. 59 (3) 635-647.

Kania, D. S., Weathermon, R. A.. Anticoagulation monitoring. Notes from clinical skills course 581.



Last updated: October 31, 2000


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