Both heparin and warfarin show great variability in response between individuals, this inevitably means that careful monitoring and dose adjustments must be undertaken, particularly in the first few days of treatment.
Check the patient’s baseline INR, APTR, U&Es, LFT, FBC and Blood Grouping before commencing treatment. If abnormal, this may need further assessment before administering anticoagulant drugs.
Patients with Atrial fibrillation/flutter should be loaded on 3mgs daily for 6 days and an appointment made for the Anticoagulant clinic on day 7.
Refer to Heparin infusion care pathway.
Warfarin is a vitamin K antagonist and has an anticoagulant effect via a reduction in the levels of vitamin K dependent coagulation factors produced by the liver (ie. factors II, VII, IX, and X). Patients should be loaded with and their warfarin dose thereafter adjusted, according to the dose schedule below.
Reversal of the warfarin effect depends on the synthesis of new factors, which can be expedited by the administration of vitamin K or their replacement by infusion of fresh frozen plasma (or factor IX concentrate which contains all of the necessary factors).
Day | INR | Warfarin Dose (mg) |
First* | <1.4 | 10 |
Second* | <1.8 | 10 |
1.8 | 1 | |
>1.8 | 0.5 | |
Third | <2.0 | 10 |
2.0 - 2.1 | 5 | |
2.2 - 2.3 | 4.5 | |
2.4 - 2.5 | 4 | |
2.6 - 2.7 | 3.5 | |
2.8 - 2.9 | 3 | |
3.0 - 3.1 | 2.5 | |
3.2 - 3.3 | 2 | |
3.4 | 1.5 | |
3.5 | 1 | |
3.6 - 3.7 | 0.5 | |
>4.0 |
0 | |
Predicted maintenance dose | ||
Fourth | <1.4 | >8 |
1.4 | 8 | |
1.5 | 7.5 | |
1.6 - 1.7 | 7 | |
1.8 | 6.5 | |
1.9 | 6 | |
2.0 - 2.1 | 5.5 | |
2.2 - 2.3 | 5 | |
2.4 - 2.6 | 4.5 | |
2.7 - 3.0 | 4 | |
3.1 - 3.5 | 3.5 | |
3.6 - 4.0 | 3 | |
4.1 - 4.5 | Miss out next days dose then give 2 mgs | |
>4.5 | Miss out next 2 days doses then give 1 mg |
* Reduce loading dose if elderly, abnormal LFTs or known potentiating drug.