INR testing techniques/methods

I have posted on here on a number of occasions my views on the two methods of INR testing. Venous Draw v Coaguchek home testing.

Firstly, to recap.When I lived in Surrey, my surgery was fully equipped with the Coaguchek XS testing devices which included the software installed on the practice computers for INR bloods analysis and for calculating new dose and next test date.

At the time I was diagnosed with AF and put on Warfarin I was working full time as a bus driver (working a variety of shifts) and so it became more and more difficult to get to the Surgery for my INR blood tests. I also travelled quite a bit to Australia and still do. I approached the surgery about acquiring the Coaguchek XS device and they agreed that it would be a big help and through the practice nurses - they would help me with training and support and also put me on their 'system' so that I could telephone in my test results. I used to telephone in my INR result from the Coaguchek device, tell them that I had not missed a dose and that there had been no variation to my medicines. They would then phone me back about 4 hours later with the INR result, the next test date and the next doseage to be followed up to the next test date. So simple.

In effect my visits to the practice for INR blood tests would cease unless I had a problem although they would like me to pop in twice a year to check the readings on my device with their device. I've never looked back, and of course, getting test strips on NHS prescription was never an issue.

Onto Cornwall .....I moved here in late July 2012.

Imagine my dismay to find that my local Cornish GP's practice only did the Venous Draw technique. Blood letting from the arm ! Sending the bloods away and waiting for 24 hours to get my INR result. I had not had this technique applied to me for INR tests and so was quite distrusting of it. Accordingly, I used my Coaguchek device to cross check the results of the Venous Draw. To cut a long story short - in the time that followed, my Coaguchek device showed some small variations in results but nothing untoward. I did however, in that time, have to challenge the doseages on two occasions and this caused my GP's practice to revise the doseage. Can't say why these errors crept in, I did get apologies though - although that wasn't the point. The point was my doseages were WRONG ! This did nothing to enhance my confidence in the Venous Draw technique. Meanwhile, I continued to use my Coaguchek device to cross check the Venous Draw and of course I used up my test strips. In November 2013 I returned to Australia for a holiday and was low on Test strips. Went and saw one of the partner GP's in my local Cornish practice and got a new pack of 24 test strips - without qualification !!

Fast forward now to a more recent Venous Draw - Test result showed 3.5. Coaguchek result showed 2.7 taken 20 minutes before the Venous Draw.Now, I am quite prepared to accept that the two techniques may not produce the same result but I was nonetheless unhappy about this variation, particularly as my new doseage was quite a dramatic change, i.e. I had to miss one day's Warfarin altogether and reduce three days doseage from 6mg to 5 mg so that the following 6 days were all at 5mg. I decided ( as I was now going to run out of Test Strips yet again ) that I'd go back to my GP and ask for a new (Aug 2014) prescription .... and I also did some research to support my request in the event that I was refused..... because I was quite determined to fight for the strips if I had to.

Now its this research bit that may be of interest to you Warfarin users, and I'll try and be brief and list the source documents below -

1) NICE (6/02/2014) published DRAFT GUIDELINES on self monitoring tests for people on long term anticoagulation - Ref: Press-and-Media Provisional recommendations - support the use of the Coaguchek XS system for people with atrial fibrillation or heart valve disease who are on long term anticoagulation therapy. Significant benefit is enables patients to be monitored more regularly; improves health outcomes; doses can be adjusted more accurately. You could also go to - a 60 odd page document on the matter of anticoagulation oral.

2) Google ...... - then go to How often should I testmy INR and Testing methods for your INR you will find the comments about Venous Draw and Finger Stick testing - both point of care and home testing very interesting. Especially the bits on 'reagents'.

3) Reference : will give more info on reagents ..... and finally,

4) self monitoring

So, at the end of the day - having done all this research I fronted up to discuss my INR results and request a new prescription of Test Strips only to have no argument at all and got a prescription for another 24 test strips just like that. We did have a discussion which amounted to my suitability to self test and that was it. Once the GP was totally satisfied that I was suitable self testing material it was all done and dusted.

Then it came to agreeing to a date for my next appointment - and I said I couldn't keep that appointment due to work committments - why don't you accept my Coaguchek Finger stick reading, I asked. I said I could phone it in on the said day and get the results the next. And to my total amazement - he agreed. So I did that and phoned in my latest reading which turned out to be 2.3 from my Coaguchek device. In effect - in 2 years this surgery in Cornwall has shifted its position on INR testing from the 'old' system of Venous Draw ONLY, to doing both Venous Draw and the 'Coaguchek' system - which also means supporting those patients on Coaguchek by prescribing on NHS the Testing Strips. Still not as advanced as my Surrey surgery but moving along in the right direction.

The one thing that is clear from my research is that the Venous draw technique is the least perfect of INR testing techniques - it is very susceptible to the quality of the commercially produced 'reagents' and to the skills of the laboratory technician. In fact, both patients and physicians are usually unaware of the sensitivity of the 'reagents' used in any given laboratory. The Coaguchek system uses only 'reagents' with the highest sensitivity available in their test strips and unlike laboratories, where this sensitivity can vary, products like Coaguchek test strips have the most consistent high quality 'reagents'. In my opinion - this means that something like Coaguchek is the more perfect, more accurate and more reliable system.

Those of you interested can refer to the source documents I’ve listed and read for yourselves.

Apologies for the length of this I'm not a jouirnalist so couldn't edit it and make it much shorter :-)


Aussie John

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10 Replies

  • At the Patients Day last year John, the specialists in Warfarin told us variable the test was and that every little difference made it's effect on the result. So storage time, how long in transit would all add up. He also said the test itself was only accurate to about 0.5 of an INR. I guess the Coag machine cuts all that out and you just get an immediate result.

    I wish GP practice would get a test machine, my arms look like a heroin addict.


  • Thanks Aussiejohn,

    Your post may well turn out to be useful for those of us who are considering self testing. I've only been on Warfarin since March but now that my INR appears comparatively stable (with 6 weekly intervals) the self testing route is looking attractive. My surgery does use Coaguchek and I did ask my GP about getting my own in March, but he was rather non-committal at the time. I will bring it up when I see him next and make sure I am armed with some of the details which you have so kindly provided.


  • No worries Cyril .... and good luck. Just sorry I had to make it such a convoluted post but it is a tedious matter and one needs to read things and think things through... I rather hope some people on here will put my post 'through the wringer' - I might be seen as a bit biased toward self testing but I can only call it as it is for me.


  • Well done John,

    I've been using my Coaguchek for about a year now. Have noticed some variations when blood sent to lab for testing but recently took my machine with me when tested at the surgery at my normal 60 day check up and both machines gave the same reading.

    My surgery in Somerset dose not recognise self testing, or provide the strips, so I just go my own way and test weekly adjusting a bit here and there to keep in range. The nurse knows I do this but what she puts in the computer I have no idea.

    I will study the links you provide and have another go at the GP in the near future. Considering the time and effort this would save the practise, I fail to see their problem, plus my own peace of mind. I certainly am not happy with 60/70 day testing as we all know the levels can vary much quicker than that.

    All the best


  • Hi Ray,

    Yeah, I've been on 70 day tests too - my feelings are the same as yours - to be honest - because I'm pretty INR stable I'd be happy with 14 day tests.


  • As an uneducated in all this, so far, I found your post really informative, thank you

  • Good stuff, Aussiejohn. Very interesting. My surgery has recently bewitched over and formed an INR Testing Clinic and unless you are experiencing wildly varying results, no longer use blood letting. My feelings on self testing run alongside yours and if I had trouble meeting appointments or visits out of the area, I would champion self testing with my GP. At the moment I am on two weekly visits as they get me stabilised on my 2.5 target. Once that happens, this period will increase and that's when I might worry a bit. I'm not obsessive with INR readings - I know they can vary - but when I had my first machine reading taken at the clinic, it was 0.9 when I had gone in expecting my usual 2.7. That was , if not scary, a bit worrying. It's climbing back up again with an increased dose. That episode gives me good reason to self test, even at my expense, to give me peace of mind.

  • Good post, thank you and I agree. I find it very reassuring to be able to self test, I get strips on prescription and I adjust my dosage without consulting anybody. I go to the clinic about every 10 weeks as a double check.

  • Hi John,

    I totally agree. However out of interest I recently had a calibration check of my monitor. As we're on the border of Hants and W Sussex, and a slight mix up with forms, they took 2 samples of venous draw which were sent to Portsmouth and Chichester respectively. My monitor read 2.7, Pompey were 2.6 and Chi 2.8. So my monitor appears spot on and the venous samples slightly out.

    Self management (i.e. self monitoring and self dose adjustment) has been shown to be most effective at maximising Time in Therapeutic Range (TTR). Keep in range 90%+ of the time and your risk of stroke drops to about half of the average anti-coagulated patient and about a quarter on non coagulated people.


  • G'day Mark,

    This indeed supports a comment in one of my links ...... that it is it is possible for blood from one patient to be sent to two different hospitals and get two different results. This was in the context of quality of reagents used by the laboratory and the skills of the lab technician.... whereas with the self testing device everything is 'locked in' to the device and test strips and the reagents are the highest quality and sensitivity.

    I don't go round 'fretting' over my INR but, like you, I do believe firmly in self testing and self management and down here in Cornwall these changes are slowly coming but hell ..... its like trying to mate elephants ..... not to be accomplished without a lot of squealing and screeching !


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