Pharmacology Notes

Regular monitoring of warfarin is mandatory. This is done using variations of the prothrombin time. The INR standardises results between laboratories by comparison with a World Health Organization standard.

The goal of warfarin loading regimens is to rapidly attain a stable therapeutic INR without over-anticoagulation. In the treatment of deep vein thrombosis, if the time to reach a therapeutic INR is delayed, ongoing UFH or LMWH therapy is required. When warfarin is indicated for prophylaxis rather than therapeutic reasons, there is less need to rapidly attain a therapeutic INR. The INR should be determined before commencing warfarin, and if it is 1.4 or greater then careful consideration must be given before initiating warfarin for any condition. A number of dosage protocols have been developed to facilitate the attainment of a stable therapeutic INR. If local guidelines exist that reflect the local circumstances—for example initiation of warfarin in the elderly—these should be sought to provide assistance. A widely used schedule is 5 mg daily for two days, with the dose thereafter adjusted according to the INR to achieve the required INR. In older patients, those with impaired liver function, and those with congestive heart failure, oral anticoagulation should be started cautiously. The patient's INR should be determined daily or every second day until it has stabilised within the therapeutic range.

Oral anticoagulation with warfarin should preferably be started in conjunction with heparin, as initially warfarin may be associated with a procoagulant state resulting from a rapid reduction in the concentration of protein C, which is also a vitamin K–dependent protein. Heparin should be continued until the INR has been in the therapeutic range for two consecutive days. Patients at a high risk of thrombosis and those with a large atrial thrombus may need longer treatment with heparin.

Patient education is an important part of warfarin management and should be explicitly planned using resources such as anticoagulant booklets. These include information on what to do when starting or stopping new medications (including over-the-counter and complementary medicines), advice about diet and alcohol intake, and columns for the date of each blood test, the INR result, and the recommended daily warfarin dose. It is also important to encourage use of the same brand of warfarin for all dosage strength