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.
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!
I’ve now been testing my INR using my Coaguchek device for five months now.
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.
This morning, I cut my hand accidentally, as I walked the Regent’s Canal. How I don’t know, but despite timely repairs by the nurse at my surgery nearby, the wound refused to stop bleeding and I had to go to A & E at University College Hospital.
The nurse at UCLH, who bandaged my hand, put the bleeding down to the interaction between Warfarin and the other drugs I am taking.
The strange thing is that I can now type easier and get the Shift and Control keys right.
As September is now finished, I can show a graph of my daily INR tests 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.
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.
As August is now finished, I can show a graph of my daily INR tests 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.
To illustrate the changes you get in INR, I’ve made a graph of my last fourteen readings.
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.
Tomorrow, it will be eleven weeks since I started self testing my INR on a daily basis. The results are here.
I should say that after I had my stroke, an eminent cardiologist said that if I got my Warfarin right, I wouldn’t have another stroke.
So can I come to any conclusions from the tests I have been taking?
I did miss one day early on, but otherwise I’ve taken the test successfully on a daily basis.
I’ve now developed a daily routine in the morning, where I do my stretching and exercises after checking my computer, then have a shower and breakfast, before doing the washing-up by hand, which warms my hands. I then take the test and only rarely do I fail first time and need a second strip.
So the first conclusion, is develop a routine for when you do your tests, that suits your personality and lifestyle.
One thing that you notice from the tests, is that there is quite a large variation between days. A change of 0.5 in the INR, either up or down is not uncommon. This is not a problem, but it could with some people worry them and then they might start to chase their target INR, by constantly changing the dose.
Hot days incidentally, do seem to try to force the INR upwards and although you won’t find this on the Internet, a medical professional has told me that it happens.
I use a very simple manual algorithm, based on my training and experience as a Control Engineer. I know from when I was living in Suffolk, that a Warfarin level of 4 mg. a day is about right to meet my target of 2.5. So I use a simple algorithm, summarised as follows.
INR less than or equal to 2.2, take 5 mg.
INR higher than or equal to 2.8, take 3 mg.
INR between 2.2 and 2,8, take 4 mg.
So how has my INR behaved?
If I look at the average value of the last 28 days, it is 2.56 and this rolling 28 day average has been within 0.1 of 2.5 for the last seven weeks. I couldn’t calculate it before, as I didn’t have enough data. An interesting figure is that the standard deviation of the readings is about 0.3. Effectively this says that nearly all of the readings are within 2.2 and 2.8, which is within my target range of between 2 and 3.
So as the patient, I think I could safely say that my simple algorithm works.
But perhaps what is most interesting is that the 28 day average for the dose I’m taking is around 3.8 mg. So rounding this to the nearest tablets, that means if I can’t take a reading for some reason, then I should take 4 mg.
So I can conclude that the daily testing has given me a very sensible daily dose, which is virtually the same, as I took, when the tests were done by nurses, hospitals and laboratories, at great expense to the NHS.
So should all those going on Warfarin be assessed to see, if they could self-test their INR levels?
I believe they should! And it’s not just me!
An organisation called the Anticoagulation Self-Monitoring Alliance is pushing for more self testing. Be cynical if you like, but it is part-funded by Roche, who make the self testing meters.
On the other hand, how many diabetics test their blood sugar levels regularly and have a better lifestyle because of it?