Evidence-based On-call

Warfarin

Starting treatment
Indications
Dosing and monitoring
Complications
Follow-up

Dosing and monitoring

Start warfarin as soon as possible ^a and monitor its effect using INR.

For patients with venous thromboembolism, give LMWH or heparin while awaiting satisfactory oral anticoagulation. ^a
Give heparin ^d and LMWH ^d for a minimum of 5 days and continue for 2 days after the INR is within therapeutic range ^d
Use a set dosing regimen (preferably by computer) ^a and seek expert advice when indicated. ^a

Here is an example dosing regimen to achieve a therapeutic range of INR 2.0 to 3.0.

^b
Give the warfarin at 5 to 6 pm, and measure the INR at 9 am the next day.
Monitor INR daily until in range and stable.
Use lower loading doses - they are more effective

Warfarin dosing regimen to achieve INR 2.0 to 3.0 ^b

Dose for age (mg)
Day	INR	up to 50	51 to 65	66 to 80	> 80
1	<1.4	10	9	7.5	6

2	<1.6	10	9	7.5	6
	> 1.6	0.5	0.5	0.5	0.5

3	<1.8	10	9	7.5	6
	1.8 to 2.5	4.0 to 5.0	3.5 to 4.5	3.0 to 4.0	2.5 to 3.0
	2.6 to 3.0	2.5 to 3.5	2.5 to 3.5	2.0 to 2.5	1.5 to 2.0
	3.1 to 3.5	1.0 to 2.0	1.0 to 2.0	0.5 to 1.5	0.5 to 1.5
	3.6 to 4.0	0.5	0.5	0.5	0
	> 4.0	0	0	0	0

4	< 1.6	10.0 to 15.0	9.0 to 13.0	7.5 to 11.0	6.0 to 9.0
	1.6 to 1.9	6.0 to 8.0	5.5 to 7.0	4.5 to 6.0	3.5 to 5.0
	2.0 to 2.6	4.5 to 5.5	4.0 to 5.0	3.5 to 4.5	2.5 to 3.5
	2.7 to 3.5	3.5 to 4.0	3.0 to 3.5	2.5 to 3.0	2.0 to 2.5
	3.6 to 4.0	3	2.5	2	1.5
	4.1 to 4.5	omit next day's dose, then 1.0 to 2.0	omit next day's dose, then 1.0 to 2.0	omit next day's dose, then 1.0 to 2.0	omit next day's dose, then 1.0 to 2.0
	> 4.5	nil: hold dose	nil: hold dose	nil: hold dose	nil: hold dose

Decrease the dose by 33% if the patient has one or more of the following risk factors:

Severe congestive heart failure (ejection fraction < 30% and/or biventricular failure)
Severe chronic obstructive airways disease (oxygen or steroid use or dyspnoea at rest)
Concurrent amiodarone use.

Expiry date: May 2003
Levels of Evidence used in grading these guides

With thanks to Drs. David Keeling, John Reynolds, David Sackett, Sharon Straus and Alan Townsend
for use of their anticoagulation guide on which this one is based.

Author	CM Ball
Reviewer	A Gallus
CAT Writer	CM Ball