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Venous v finger orick

Gradually Warfarin / INR will be taken on at GPs using Coaguchek.

I have a Coaguchek and there is always a slightly different reading to the venous sample.

I am feeling rather edgy about this changeover.

Comments grateful received.

Susie

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Don't feel edgy about it, just factor in that generally the coaguchek will read under the veinous in or around 0.3 - 0.5. Your INR is not set at an exact figure but between two therapeutic levels, it gives you ample leeway.

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I am correcting myself, please read that generally the coaguchek will read over the veinous...

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There often are differences. Give the same venous blood to two labs and you could get differences between them. Take two tape measures and measure a table and you could get differences. For venous blood between labs, a difference of 0.3 would be normal, and it could be more. It is similar for comparing Coaguchek against venous blood.

The medical profession is understandably nervous about the various home-check systems on the market, so studies have been done. It is in the interests of the companies making the machines and strips to keep the standards high, knowing that they are regularly checked against venous blood. The strips and the machines have checking mechanisms built in.

Personally, I would only be concerned if the difference was 0.5 or more. And I would suspect the lab as much as the Coaguchek machine/strips. I would also repeat the test with a new batch of strips before questioning the machine.

The good thing about Coaguchek is that it is possible to use a completely different method for comparison -- to compare with venous blood. People using only venous blood do not usually have the possibility of double checking with another lab.

What matters is that the two results are roughly right. Then stick with one lab or one machine. Notice differences, not the actual figure.

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One of the few things I remember from school physics is that we must always assess the accuracy of our measuring tools. This is not difficult. Abroad, I have met people who never went to secondary school who are fully aware of this. They know for instance that if I want a new window made, then they had better come to my flat to measure up with their own meter, because my meter would probably be different.

Yet, on a quick look at the medical literature, some articles assume that venous blood measurements are universally accurate. The reality is there is no such thing as a widely available gold standard in normal INR measurement.

If you can stomach it, this is one of the better articles. One comment is highly relevant to this thread.

"....When a measurement on the patient's POCT coagulometer is different from the that of local laboratory, which one is the 'correct' INR? This is not known. " Page 256. Christensen TD, Larsen TB. Precision and accuracy of point-of-care coagulometers used for self-testing and self-management of oral anticoagulation therapy. J Thromb Haemost 2012; 10: 251–60.

There is equal doubt about the coagulometer and the venous blood. Hmm. How many health professionals know that!! Which brings us back to common sense.

1/ Only be concerned if wildly out, say more than 0.5

2/ Stick to one system only

3/ Each system is presumed to be OK to report change. Rely on that more than the actual level.

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Our GP practice has used coaguchek for at least 6 years. It is so much easier to get an instant print out of result and dosage. Any variation is negligible and if you were retested the next day would be different anyway.

The only slight drawback is that the software tends to be in the present and does not take in trends by looking at previous results ( or what is your normal dose) but our nurses pick up on that and can grab a GP to override the computer dose if necessary.

I was sad to lose my contact with the care assts who run the clinic and are very knowlegeable. I now take Apixaban

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Most surgeries have been using coagucheck for many years now so don't worry. A few years ago at HRC we had a presentation from the professor who is top in UK on INR testing and he told us just how inaccurate lab testing was. Time on shelf was a contributing factor amongst other things so the sooner done the more accurate. Hence Coagucheck is generally more accurate than your practise taking blood and sending it to the hospital for testing.

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Mine is taken by thumb prick, and only ever by full blood check if needed for cardio version or other procedure. My clinic at Chelwest is very efficient, but I suppose GP would be nearer. It keeps changing. When first on warfarin went to Chelwest, then changed to GP hub, not mine, and then went back to Chelwest

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Our phlebotomists nearly always tell me what to do based on the software, then give long intervals for check up. This has thrown up huge anomalies, so I just ignore the advice. I wouldn’t attend if given the choice. Yesterday the phleb told me she wasn’t interested in my measurements at home. I wonder how often patients at this surgery are in range.

I think one of the fundamental reasons that Warfarin management generally compares badly to DOACs and why the trials are also against Warfarin, is that there are insufficient resources for proper monitoring. Does anyone have experience of being monitored by a pharmacist?

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NICE supports home testing. Many CCGs are less keen.

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NICE also recommends self adjustment of Warfarin as long as an individualised algorithm is agreed and the patient is “cognitively capable” and sticks to the agreed dosing schedule. Which I guess rules me out 🙂, subject to individual GP discretion.

CCGs may have in mind medicolegal issues.

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That makes sense. The testing is one thing, the interpretation of the results is another. I personally think a pharmacist is well placed to understand what is going on and to do the dosing. The old system of yellow books in this sense was much better. A good doctor could look through previous results, going back several years if needed, and give dosing advice based on many factors.

The best dosing method I have seen so far is Kim Y-K, Nieuwlaat R, Connolly SJ, Schulman S, Meijer K, Raju N, Kaatz S, Eikelboom JW. Effect of a simple two-step warfarin dosing algorithm on anticoagulant control as measured by time in therapeutic range: a pilot study. J Thromb Haemost 2010; 8: 101–6. onlinelibrary.wiley.com/doi...

I also as a patient have my own refinements to this, just accepted for publication but not out yet.

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Will you let us have sight of that when appropriate? Alternatively, where are you publishing?

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It is a tiny journal, but it is peer reviewed and is open access, so, following the custom of many journals I will put it up on my website before the end of the month, marked as a "pre-publication" edition. I am not sure how much I am allowed to blow my own trumpet on this site, hence my caution.

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It’s just like using the same scales to get weighed you get different readings on different scales

so once you start using one system the result will be consistent, lol well as consistent as Warfarin can be

X

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Good comparison. I am always looking for common examples like this. Thanks.

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The difference between lab test and Coagucheck test it the same for Inr 2.5 till 3.00,.Bigger the numbers of INR, bigger the difference.If Coagucheck says 3.5 than the lab test is propabily 4.Coagucheck give to me the possibilities of checking often( every 4-th day) so I can reduce or increase the blood thinner.

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Hi Susiebelle,

For 2 years prior to moving to Cornwall I used Coaguchek and my practice in Surrey supported me in this totally. Brilliant.

The first practice I registered with in Cornwall used the venous draw technique, however, I never really trusted this. Every time I had a venous draw for my INR I carried out a Coaguchek test just prior to going to the Surgery. Coaguchek and venous draw results were never, ever the same ...... nor did I expect them to be ...... nor did it worry me. Until I became suspicious about the dosages of Warfarin I was being prescribed. The doses were always being varied. I simply used Coaguchek as the mechanism by which I could monitor myself.

I repeatedly asked this surgery to support me with Coaguchek ....... they refused.

Time went on and twice I had to go back to this practice and challenge them ...... not on the venous INR reading per se, BUT, on the new dosage of Warfarin I was prescribed as a result of these venous draw readings. AND twice (thanks to Coagucghek) my GP had to go away and check everything and ring me back and confirm a new dosage !! The original was wrong.

In the end I gave this surgery the sack, found another locally who does support Coaguchek and I've never had a problem since. I never found out who prescribed the warfarin doses arising from the Venous INR's, the lab or someone interpreting the lab results in the original Cornish GP surgery - but - it was totally unsatisfactory and unacceptable to me.

Sure, warfarin being warfarin, there are fluctuations in INR and if these are too dramatic I simply use green veg as the mechanism by which I can stabilise the INR.

John

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