Warfarin Initiation

Warfarin is a very individualized medication without a “standard” dose. In order to optimize the therapeutic effect and minimize complications such as bleeding, close monitoring of the degree of anticoagulation is required.

Outpatient Warfarin Initiation

During the initial stage of therapy, expect to check the INR frequently. The intervals between tests can be lengthened if the patient maintains stable target-range INR levels on an unchanged warfarin dose.

The process includes, checking baseline labs:

  • INR
  • CBC with platelets
  • Liver panel
  • Albumin

Also consider beginning with lower initial doses for:

  • The elderly
  • Malnourished or debilitated patients
  • Patients with heart failure
  • hepatic insufficiency
  • Patients at higher bleeding risk
  • Concurrent therapy with P450 inhibitors

Estimated Starting Doses

  Black White/Hispanic Asian
Age <70 7.5mg 5mg 2.5mg
Age 70+ 5mg M: 5mg
F: 2.5mg

Ideally, INR should be checked within 3-5 days of the first warfarin dose. Steady state is achieved in 10-14 days.

Subsequent Dosing

INR 1.0-1.1 1.2-1.5 1.5-1.9 2.0-2.5 2.6-3.5 >3.6
Dose Change Increase 100% Continue Same Consider Decrease 10-20% Decrease 25% Decrease 50% Hold 1-2 days and recheck INR
Days to next INR 3-5 5-7 5-7 5-7 5-7 2

Other options: http://warfarindosing.org/Source/Home.aspx

After a dosing regimen is established, it is advisable to consolidate to a single strength of warfarin.  While not problematic in select patients, use of multiple tablet strengths can lead to confusion, medication errors and adverse events. 

Inpatient Warfarin Initiation

UC San Diego Inpatient Warfarin Initiation found through Web Ref provides a detailed daily guide to warfarin dosing.