Differences in Warfarin readings

How is it that differences in readings with different equipment are not taken much notice of ? I have read of people having to go to hospital to have their inr recorded regularly. It made me think about the time between the reading and the recording as my cougachec machine gives a specific time space to record the result and if you get that wrong it just shows error . Having had a bad bleed years ago I now have my inr checked at intervals with the surgery who have now got cougachec too. Perhaps it doesn't matter too much , but it did to me. Be well and safe, Terry

9 Replies

  • A few years ago at HRC we had a talk from an expert on INR testing who explained why it was varied and how two labs may get different results from the same sample. That talk came out of a discussion on the old Yahoo forum about the spread of results between Coaguchek and Lab testing which we worked out by vote to be up to 0.5 but generally about 0.25.

    In principle Coaguchek should be more consistent as the sample is fresh and the machine uses the same chemicals each time. Despite this he did think that in order to be sure of being covered we should aim for an INR between 2.5 and 3.5 rather than 2 to 3. This was never incorporated into any guidelines and was just his take on things. He also stated that there was little additional risk up to INR 5. although my experience of local nurses is that they tend to panic over 3.5. I don't.


  • Interesting information on the readings Bob.

    I am still at the stage of adjusting warfarin intake to settle on an INR of around 2.5. My check last week gave 3.8 and the INR Sister didn't seem too concerned just reducing my daily dose to compensate.

    I was in need of more rat poison and when I took my script to the pharmacist she asked if I'd had a recent blood test and what the result was. I wouldn't say that she panicked but her higher level of concern was obvious....which in turn caused me a heightened level of concern!

    I'll utilise your approach in future.

  • I've been trying for year to get rid of the rat poison tag and there you go!

    A rodent operative of my acquaintance told me it hasn't been used for rats for many years. Simple reason. It was given on grain which was rich in VITK and was self defeating.


  • Oops, sorry. My bad!

  • I'd not heard about the grain before. The rats must have loved it.

    We lived in a block of flats and I spotted a rat sunning itself on the steps of the communal garden area. I phoned the council and they sent the rat man down. He searched the area and found where they were nesting and said that he would put some stuff down and I would have to pay £36.

    Outraged I said that it was not my rat but a communal one. He laughed and phoned his boss and the charge was dropped.

  • My inr was 5 when I was taken into hospital with the bleed around my liver. It was within the first three weeks of taking warfarin. The dr who saw me said they usually saw bleeds like that in bike riders who hurt themselves on the handlebars in an accident. I hadn't been doing anything to cause it to happen. Just bad luck I suppose but extremely painful. Thanks for your reply be well Terjo

  • Well said. "Little additional risk up to INR = 5. "

    In addition, a low INR is much more serious and risky than a high INR. This is because a low INR increases the risk of clotting ie increases the stroke risk, and a stroke is always serious. A high INR increases the risk of a bleeding event being serious, but bleeds are usually safer than strokes.

    Many medical sources will say that even INR up to 9.0 is not particularly dangerous, and can be handled by the self dosing patient without intervention of a doctor, unless of course there are any other risk factors or clinical signs. My favourite reference on this is Kim et al 2010. Effect of a simple two step warfarin dosing algorithm on anticoagulant control as measured by time in therapeutic range: a pilot study. Journal of Thrombosis and Haemostasis vo8 pp101-106. It is an easy read, and the table on p103 is clear.

    In addition, there is 'regression towards the mean" therefore high readings tend to autocorrect without intervention. In fact, intervention ie a dose change can make it worse since it creates the dreaded self amplifying swings.

  • Three times I have "calibrated" my coaguchek against the coag clinic Venus tests and each time it has been 0.1 difference. Therefore it is pretty consistent.

    Round here they won't prescribe coaguchek test strips because we have a coagulation service covering one third of the county and my local clinic is under two miles away (nearer than GP) and there are no parking charges.

  • My GP gave me a repeat prescription for the strips . They are available on the NHS and lessen the pressure on the surgery. I'm fortunate. Be well Terjo

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