Anticoagulation Service For Health Professionals

Guidelines for Initiating Inpatient Warfarin

Loading doses of warfarin (e.g., 10 mgs on days 1 and 2) are no longer recommended.
Several controlled studies have shown that starting patients on 5 mg of warfarin daily achieves a therapeutic anticoagulant effect as rapidly as 10 mg loading regimens while causing fewer supratherapeutic INRs. We suggest the following algorithm by Crowther et al (see Arch Inten Med 1999; 159:46-48 and Ann Intern Med 1997;127;332-333) to guide the dosing of warfarin during the first several days of therapy. It is particularly useful in hospitalized patients in whom lNRs are typically checked daily. It may not be practical in patients who are started on warfarin out of hospital, since INRs are not typically checked daily in this setting.

This algorithm should not be used in patients taking Amiodarone, which potentiates warfarin. Such patients typically require lower initial warfarin doses than those shown in the algorithm.

Suggested Nomogram for Starting Patients on Warfarin*

INR Dosage
DAY 1 A baseline INR should be obtained prior to starting warfarin 5.0 mg**
DAY 2 <1.5
1.5 - 1.9
2.0 - 2.5
> 2.5 		5.0 mg
2.5 mg
1.0 - 2.5 mg
0.0 mg
DAY 3 <1.5
1.5 - 1.9
2.0 - 3.0
> 3.0 		5.0 - 10.0 mg
2.5 - 5.0 mg
0.0 - 2.5 mg
0.0
DAY 4 < 1.5
1.5 - 1.9
2.0 - 3.0
> 3.0 		10.0 mg
5.0 -7.5 mg
0.0 - 5.0 mg
0.0
DAY 5 < 1.5
1.5 - 1.9
2.0 - 3.0
> 3.0 		10.0 mg
7.5 - 10.0 mg
0.0 - 5.0 mg
0.0
DAY 6 < 1.5
1.5 - 1.9
2.0 - 3.0
> 3.0 		7.5 - 12.5 mg
5.0 - 10.0 mg
0.0 - 7.5 mg
0.0
*This algorithm is compatible with heparin (either unfractionated or low-molecular-weight).

**In selected patients (e.g., very large individuals or those on medications known to antagonize warfarin), a Day 1 warfarin dose of 7.5 mg may be appropriate.

Other important points regarding initiation of warfarin

  1. Please consult us when beginning warfarin therapy. This gives the Anticoagulation Service staff the time needed to coordinate patient education and to arrange for monitoring of INR's as an outpatient. If the patient is admitted on a weekend, a message can be left on our answering machine.
  2. Please write initial prescription with 2 mg or 2.5 mg tablets (e.g., prescribe 5 mg/day dose as "warfarin 2.5 mg tabs, take 2 q HS"). This greatly facilitates the ability of Anticoagulation Service nurses to make minor dose adjustments based on follow-up INR values.