Our Website

Check out our website for even more up to date information!!

Wednesday, October 31, 2012

Are you happy with the advice you get on changes in warfarin dose when your INR is not right?

I have some major concerns about the advice that I receive from my pathology lab when they indicate that changes to my INR are required.

My INR target range is 2.0 to 3.0. Now if you have read the 2 posts below, you will realise that the measurement of INR is not particularly precise, and that adds to the difficulty of assessing just what changes are required.

A few weeks ago, I was on a schedule of 4.5 mg and 5 my alternative days. On testing,  my INR was below 2.0, at about 1.7.The pathology lab advised that I should now take 5 mg warfarin on Sunday, Tuesday  Thursday and Saturday, and 4.5 mg on the remaining three days each week.

Now on my original schedule  I would get 7 days of 5 mg and 7 days of 4.5 mg each fortnight, making a total of  66.5 mg per fortnight, or a total of 33.25 mg/week.

On my pathology labs new schedule, I would get 4 days of 5 mg per week, and 3 days of 4.5 mg/week, or a total of 33.50 mg/week.

So, although my INR was significantly below 2.0, at 1.7, the dosage suggested increased my warfarin intake by only 0.25 mg/week, or about 0.75% !!!!

My INR of course did not shift.

Again, quite recently, my INR was 2.0, and the lab called to change my dose.

I had been on 4 mg and 4.5 mg alternate days, a total of 59.5 mg/fortnight, or 29.75 mg/week.

Their new schedule they suggested was 4 mg on Sunday, Tuesday,  Thursday and Saturday, and 4.5 mg on the remaining three days each week. This resulted in a dosage of 29.5 mg/week.

So the dosage suggested was in fact a decrease of 0.25 mg/week or about 0.75%, in spite of the fact that my INR was at the lower level recommended!!!

I rang to point this out, and they changed my dose, somewhat reluctantly, to 4.5 mg Monday to Friday and 4 mg on Saturday and Sunday. This results in a total dosage of 30.5 mg/week.

So their new recommendation was an increase of 1.0 mg/week. or 3% increase, compared to their first recommendation of a decrease of 0.25 mg/week or a decrease of 0.75%.

Clearly, I have no confidence in the information or recommendations provided by my pathology lab.

Now let's have a look at what is really required for changes in INR dosage to have a real effect. Below is part of an abstract from an article published in 2000.

Management and dosing of warfarin therapy.

Division of General Medical Science (BFG), Washington University School of Medicine, St. Louis, Missouri, USA.
The American Journal of Medicine 

When initiating warfarin therapy, clinicians should avoid loading doses that can raise the International Normalized Ratio (INR) excessively; instead, warfarin should be initiated with a 5-mg dose (or 2 to 4 mg in the very elderly). With a 5-mg initial dose, the INR will not rise appreciably in the first 24 hours, except in rare patients who will ultimately require a very small daily dose (0.5 to 2.0 mg). Adjusting a steady-state  warfarin dose depends on the measured INR values and clinical factors: the dose does not need to be adjusted for a single INR that is slightly out of range, and most changes should alter the total weekly dose by 5% to 20%. The INR should be monitored frequently (eg, 2 to 4 times per week) immediately after initiation of warfarin; subsequently, the interval between INR tests can be lengthened gradually (up to a maximum of 4 to 6 weeks) in patients with stable INR values. 

I have underlined the important text - most changes should alter the total weekly dose by 5 to 20%!!! What is going on with 0.75% or so advised by my lab - this would be the same effect as eating a bit more greenery in the diet!

The figures advising changes in the range 5 to 20% for useful changes in INR are supported by a number of other works.

So my advise to you, if you are on warfarin:

  • ALWAYS ask for your INR when you get tested, don't just blindly accept the dosage changes recommended
  • Keep records of your INR and dosage and dates tested
  • If changes are recommended, do the arithmetic yourself, and see just how much of a change in your weekly dose  has been proposed - if it is less than 5%, ask why!

Just a bit more information supporting these recommendations from

>10.0Stop warfarin. Contact patient for examination.
7.0-10.0Stop warfarin for 2 days; decrease weekly dosage by 25% or by 1 mg/d for next week (7 mg total); repeat PT³ in 1 week.
4.5-7.0Decrease weekly dosage by 15% or by 1 mg/d for 5 days of next week (5 mg total); repeat PT in 1 week. 
3.0-4.5Decrease weekly dosage by 10% or by 1 mg/d for 3 days of next week (3 mg total); repeat PT in 1 week.
2.0-3.0No change.
1.5-2.0Increase weekly dosage by 10% or by 1 mg/d for 3 days of next week (3 mg total); repeat PT in 1 week.
<1 .50=".50" td="td">Increase weekly dose by 15% or by 1 mg/d for 5 days of next week (5 mg total); repeat PT in 1 week.

I'd suggest that ytou use these as an indication of what should be suggested by your pathology lab

No comments: