INR Results Of A Coeliac Using Warfarin And Taking Terbinafine
This graph shows my INR a period between the 20th of May and the 25th of June.
I should say that I have a degree in Control Engineering from Liverpool University.
My aim here is to keep my INR between two and three, with a target value of 2.5.
Since starting to self test, I normally take around 4 mg. a day of Warfarin, but I have found that five is a better dose for when I’m taking Terbinafine, which has been prescribed by my GP for a fungal infection. The drug is well-known to affect the action of the Warfarin.
So now I take 5 mg. unless the INR is 2.8 or more. In which case I reduce the dose from five to four. On the other hand, if the level is 2.2 or below, I increase it to six.
The average INR value for the period shown was 2.6 with a standard variation of 0.2.
The peak at the beginning of June may have been caused by a B12 injection or hot weather. Both of which seem to raise my INR.
You will notice that the INR went up around the beginning of June. I can’t be sure, as I don’t have the dates, but this may have been caused by having a B12 injection.
Interaction Between Warfarin And Terbinafine
A few weeks ago I was prescribed a course of oral Terbinafine to clear up a fungal infection. It seemed to work well, except that there is still some of the tinea in my toe-nails.
But over the time, I was taking the drug, I have felt that my INR was constantly wanting to slip downwards towards and below two.
Luckily I test my INR daily, and use a simple control algorithm to calculate my Warfarin dose. Normally, it is 4 mg, but if it goes below 2.3, I increase it to 5 mg, and if it goes above 2.8, I reduce it to 3 mg. So the algorithm got me taking a lot of 5 mg doses as opposed to the usual4 mg.
Only since I finished the course of Terbinafine has the INR stabilised around 2.5, which is my target value.
I didn’t at first see any link until everything settled after the course finished. But I decided today to type “Warfarin Terbinafine interaction” into Google. I found this paper from the BMJ entitled Drug points: Serious interaction between warfarin and oral terbinafine.
I think this minor incident shows the value of regular INR testing! Because I was testing daily, as the INR started to drop, my algorithm told me to increase the dose to 5 mg. In fact my average dose has gone up from 4 to 4.5 mg. in the period that I was taking the Terbinafine.
So there was no harm done at all!
My INR For January 2014
As January is now finished, I can show a graph of my daily INR tests for January 2014.
My INR For January 2014
The average INR for the month was 2.6 with a standard deviation of 0.2. This is well within the range of 2 to 3 and just above the target of 2.5.
I’m using a simple algorithm of 4 mg. normally, with 3 mg. if the INR is above or equal to 2.8 and 5 mg, if it is below or equal to 2.2.
It would be interesting to see if the results with the switch limits set to 2.1 and 2.9, or if a little bit of integral control were to be introduced. As with all control systems, getting everything stable always needs a bit of fine tuning.
Five Months Of Daily INR Testing
I’ve now been testing my INR using my Coaguchek device for five months now.
INR August-November 2013
I’ve missed very few days.
Nothing worries me about the results, but suppose you were testing every two weeks or so, you might start to get the impression your INR results were not what they should be.
I’ve now got enough data to start doing some serious analysis.
My INR For September 2013
As September is now finished, I can show a graph of my daily INR tests for September 2013.
My INR For September 2013
The average INR for the month was 2.4 with a standard deviation of 0.2. This is well within the range of 2 to 3 and just below the target of 2.5.
Note the drop in INR starting at the 19th. This was when the weather started to get colder and fresher. The lowest value of 1.9 on the 27th was after a particularly cold night.
Compare this graph with previous results for August 2013.
My New Tab Device
I make notes as I go through the day. I also like to keep my latest INR with me. So I’ve now got a new tab device.
My New Tab Device
I use a new one every day and they cost under a penny each from Ryman.
A Coaguchek Failure
I get on well with my Coaguchek, but I did have a failure in Sweden.
The batteries chose to run out and I then had to reset the device.
Unfortunately, it is not an easy process to do without the manual, which I deliberately had not taken.
An ideal device would have a quick setup, where it took defaults for everything. After all, you always write down all your results and don’t rely on things like the date set into the device.
Everything should be simple and intuitive. It isn’t and the manual is needed too often. It also just gives you error numbers, rather than a proper error message.
The outcome was that I missed one of daily tests. Not important for me, as I just took the average Warfarin dose of 4 mg.
My INR For August 2013
As August is now finished, I can show a graph of my daily INR tests for August 2013.
My INR For August 2013
The average INR for the month was 2.5 with a standard deviation of 0.3. This is well within the range of 2 to 3 and the average was spot-on the target of 2.5.
Having once been told by an eminent cardiologist, that if I got the Warfarin level right, I probably wouldn’t have another stroke, I try to make sure I get it right.
What is interesting is that my average Warfarin dose for month is exactly 4 mg. a day.
The more I look at these results, the more I believe that daily self-testing is the best way to control INR.
My INR Ups And Downs
If you look at my INR Results, you’ll see that over the last few days from the 12th to the 20th of August my INR has swung up and down like a yo-yo. The nine readings in order have been.
2.6, 2.5, 2.7, 2.6, 2.3, 2.6, 2.2, 2.1, 3.0
The average though over that period is almost 2.5, which is my target and I’ve taken an average of 4 mg. of Warfarin. So there’s no obvious reason for the variation between 2.1 and 3. My gammy left hand too, has been a bit variable in its ability, although, it’s probably working as well as it has since the stroke.
What worries me though, is suppose you’re having your INR tested every three weeks or more in the traditional way. From my experience, you could get any number you fancy, all of which will be in the required range and you might then end up on a dose that is not the best for you.
I feel a lot happier, that I have control of my testing and dosage.
Variations In Daily INR
To illustrate the changes you get in INR, I’ve made a graph of my last fourteen readings.
INR 1-14 August
As you can see the level goes up and down, but stays within the limits of 2 and 3, with an average of about 2.5.
The Anonymous Widower 