AF Association

Home monitoring and adjustment of Warfarin dose

I have taken warfarin for 8 years. For 6 of those years I have measured my INR at home. For 2 years I have with the tacit agreement of the GPs Warfarin clinic, adjusted my Warfarin dose myself.

This has meant I can anticipate changes in INR when starting new meds or if I am unwell. I have learned about the things that impact my Warfarin dose and can achieve decent INR control without too much effort.

Much as I would if I was taking Insulin. I recommend it to you.

It's all about taking back control Aunt Ethel. Just go easy on the alfalfa and those multivits. If you insist on eating a sackful, then do make sure you eat that sackful every day Auntie. The alfalfa that is.

Baaa....humbug, said Auntie.

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Well now for me its been no more Warfarin ,about 2 years ago i was having trouble with the visiting phlebotomist at my Doctors as they kept on misrouting my blood samples. So i would get a call from the clinic to say that i had not had my INR checked when I had.

So the consultant at my local hospital suggested that i go on Xaralto 20mg rivaroxaban . Its the best thing as now only have too have a blood test every 6 months.

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Thank you Honeywine

It's true, there are many here who are happier with their NOAC (or DOAC as we are supposed to say now) than they were on Warfarin. And not many who travel the other way.

But there is likely to be a large contingent happy on Warfarin and my post was aimed at encouraging folk to try home monitoring. I use a Coagucheck.


It has been suggested that I think about taking Xaralto. Do you have any side effects and does it worry you that it has no antidote? I just can't decide what to do.


I agree with you Honeywine. rivaroxaban can not be beaten. Took it for 6 years now and it has worked a treat. Blood test every 6 months, able to live my own life again, going on holidays, etc, can drink beer/wine, no back and forth to clinics. If I have to visit a dentist I just miss for one day and it is out of your system. Carry a card in case of accident. I have had operations and no problem. I suspect that a lot of GP surgeries try to off put patients from rivaroxaban because of the cost to their particular surgery but I think on the all it saves the NHS money because there is hardly any need for appointments taking up the time of nurses, doctors, etc for regulating the doze unlike warfarin.

Best of luck to everyone and a happy New Year


I'm with Badger on this, Coaguchek and self management are the way to go on warfarin. It helps you take back control of your condition.

For Patricia, I'd suggest getting a Coaguchek. If you can get 70%+ of your readings in range then warfarin is just as good if not better than the DOACs.

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I think I would have done far better with Warfarin had I been self managing. There seemed to be a reluctance in all the regulations to acknowledge patient intelligence and I think it would have been a real battle to have grasped control.

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From what I have seen of published reasoning for handling INR by health professionals, I have found most of it to be, frankly, primitive. There are are least two problems: they have a narrow target range, usually one INR unit eg 2.0 to 3.0. My experience is that the natural biological range is nearer 2.5. I can document that. Plus, the testing equipment has low "validity" and can vary up to 0.5 from one machine to another. The threshold of protection is INR=2 therefore, the lowest acceptable INR is 2.5. When you then add in the biological range, then you end up with a "do nothing" range of 2.5 to 4.5.

The second problem is that the standard solution for too high INR is dose reduction, possibly "miss a day then dose reduction". This is crazy, because it fails to distinguish between three major reasons for a high INR. It also is the main cause for over-correction, and the dreaded swings. These major reasons are:

1/ Chance. Nature will self correct. Regression towards the mean. Homeostasis.

2/ Think of a reservoir. The level in the reservoir needs adjusting. For this you "Step" ie reduce or increase the dose for a few days, then go back to the normal dose.

3/ Your body has changed the input=output. This means that the reservoir level needs adjusting AND then a dose change.

What I usually do when I get an out of range (below 2.5 or above 4.5) result is NOTHING. I test again 7-10 days later. Then I Step. Only if stepping fails, I resort to method three.

The other matters are

a/ I take a low INR much more seriously, and would usually not wait for nature, but would nudge it higher, by an extra mg for 3 days, then normal dose.

b/ at all costs, act slowly and in small steps, to avoid swings

c/ Dose change for a high INR is LESS than for a low INR. This is because the dose/INR curve is not linear.

Self dosing allows me to take sensible decisions instead of being bound by simplistic reasoning of some, but not all, medics. Of course, if you know of other factors such as illness etc you can add that into your evaluation, which a health professional may not know, or may feel they can ignore.


That is very interesting ILowe, and extremely helpful.

Can I ask you to clarify a few things.

What is, and what do you mean by, the "natural biological range" as you would define it?

Do you have confidence that testing say a Coagucheck against reagents supplied by Roche when the clinic get them in every 6 months, addresses the validity issue?

Could you explain again why the upper limit of your do nothing range is 4.5 ?

Why are swings in INR "dreaded" Are they in themselves dangerous?

In your reservoir analogy, I understand you if it rains very hard for a day. But what if it rains 24/7 for a month. As in, I have to take Septrin ( Cotrimoxazole) for a month which seems to destabilise the INR?

When I change/add a new medication, I anticipate the likely interraction and adjust the dose.

Do you have a feel for an upper limit of INR which is important? I have not had any bleeding problems up to INR 4.0, which is the highest INR I have had.

I suppose the critical thing is what is happening with clot formation in say the left atrium. Is the INR an adequate proxy for determining that?

Are the software programs for determining Warfarin dose, used by the INR clinics now, more satisfactory than the old analogue algorithms?

Thank you for taking the time to put together such a well crafted response. A must read for those monitoring their INR at home.

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Can I draw Warfarin taking members attention to ILowes excellent post in Badgers thread. He makes some bold statements and I am hoping he will come back to expand on them.

There may be a conversation to be had on this.


Q1. What is, and what do you mean by, the "natural biological range" as you would define it?

A1. Many things in nature, including drug levels, go up and down, over various time scales. So there are swings, naturally. Hence, for a constant dosage over time, there will naturally be low points, and naturally be high points, all without changing the dose. I can document that, for myself, as a self doser for over 24 years, who has a very relaxed view about dosing, ie I am not afraid of a low figure and I am not afraid of a high figure. I have found that by doing nothing when the INR is unusually high or low, the next test result (at least 7 days later) is usually within range, and sometimes is the opposite. For instance, going from 2.2 to 4.4 then 3.5, all without changing the dose.

Q2. Confidence in Roche Coaguchek. Yes, I have as much confidence in them as most labs in UK, which also vary between themselves. In other words, lab tests and Coaguchek are not working to the same precision as say your 30cm school ruler or even you electronic scales for vegetables. The tests are more comparable to the old analogue scales that you stand on, and you are never quite sure to a kg, and your scales are not the same as your neighbours scales. There have been studies done on this. The other benefit of Roche is that the "reliability" is high, using that word in the technical sense of repeatability. So an change of 1.0 INR genuinely is a change of 1.0, whatever the starting point.

Q3. Could you explain again why the upper limit of your do nothing range is 4.5 ?

A3. Several reasons. Twenty years ago heart valve patients commonly had their target range set at 3- 4.5. There is plenty of argument out there, here on this forum and in published articles which I know of, which say that anything up to about 5.0 is, for most people, not dangerous.

Q4. Dreaded swings. I am aiming for stability, to dampening down the swings. Stable results, even a little high or a little low, are preferable to swings. Once swings start, you can go from one extreme to another, and each time the dose change seems to swing you even further to the other extreme. OK. Thought experiment. Suppose I am aiming for 2.5 to 3.5 and I am stable on 5mg. One day I get INR=4.0. So I miss a day, and restart on 4.5mg. Two weeks later I get INR= 2.0. So I increase my dose back to 5mg. The next reading could easily be more than 4.0 if swings have set in. If this continues, then dangerous lows and dangerous highs are possible. That is the way it often works in nature.

Q5. Rains hard for a month. And, Adjust when taking new medicine.

A5. These questions falls into known reasons for a change. You adjust accordingly, then go back to normal once the medicines are finished, with the caveats that the medicines may initially change the INR quickly, but change back is slow (eg Amiodarone) AND your future stability dose may be different.

Q6. Upper limit for INR. See A3. Ask if you want documentation. I have seen research papers which say that there is no real risk even for an INR as high as 9, though you can be sure that if I got such a figure I would NOT try to see if it was chance. I would miss at least 1 day, and would probably take some vitamin K by mouth. The medics are moving towards suggesting this as standard treatment for self dosers.

Q7. I suppose the critical thing is what is happening with clot formation in say the left atrium. Is the INR an adequate proxy for determining that?

A7. Clot formation anywhere is minimised by higher INR. In addition, for those with metallic heart valves, Warfarin gives more protection than the NOACS.

Q8. Software programs

A8. I do not have any experience of them. However, I once talked with an INR nurse who used one. I have also sometimes read remarks about them. The key limitation seems to be that most of them only take into consideration the previous dose, instead of the wider picture. Of course, they also fall foul of my comment about biological variation -- which will be different in each person, and will change in each person over time. It is highly unlikely that the program writer will accept the wide variations I accept.

Also, those using them seem to have limited techniques for handling out of range results, and do not distinguish between out of range (low end), and out of range ( high end).


Thank you again ILowe , I will try to process this and maybe come back to take you on. Hopefully there are scientists on board who like you, know a lot more about this than I do.

In the meantime, I think it is only fair that you admit you are a PhD and Professor of Mathematics at Imperial College London 🙂


Maths, Imperial College. Nope. I have a PhD, yes. More important, I have a B.Sc in Human Biology, I was a science teacher, and now I lecture on Research Methodology at the University of Tunis. I am paid to explain the complicated in simple terms.

But the principles I have set out I hope are clear, and workable by anyone. I have discussed them with other scientists.


An impressive CV.

And yes "explaining the complicated in simple terms" is very evident. I can think of a few here who would debate as equals with you, but maybe not so many of them are taking warfarin. 🙂


Thanks for the compliment, but I feel it is undeserved. I am only one of many. Numerous people on this forum think long and hard about their condition, over many years. They develop an expertise, and a feel for the subject that some medics lack. This is fine, when the medic recognises that the patient has a contribution to make. Several doctors have encouraged me over the years to take charge of my health problems, and to know them better than a doctor. But whenever I face a doctor, they are usually upset that I want to be treated as a peer, or at least, a junior colleague. The statements I made in these posts derive from over 24 years of self-dosing. The only explicit training I had at the beginning was a two minute discussion with a doctor, who compared warfarin dosing to charging a paintbrush. That was it. I was on my own. Glorious freedom!!

Sadly, it it is not obvious how patients can publish their ideas, so as to inform the medical profession, so, much of the expertise is found in forums like this.

By the way, your pushing my image of the reservoir by asking about a steady wind for a month, was *extremely* helpful in developing the image. You see, it is common for scientists to take an analogy/image, which is a simplification of a complicated phenomenon, and use it to make testable predictions.

It is not rocket science to ask of an "out of range" result, is it chance, is it reservoir level, is it the input=output that is the cause. The technique of "stepping" is an under used step, before deciding to change the input dose. It is also not rocket science to first establish the natural biological range, before dreaming of imposing an un-naturally narrow range. To my knowledge, the medical profession has never done that, and it would probably nowadays be considered unethical to try. This is where the independent types like me, the self-dosers come in. It would be ethical for a medical researcher to systematically ask how self-dosers -- especially those with rare/no contact with authority -- do their thinking.


When you talk of people on here knowing more than some doctors you are not wrong my last GP is about to cop a huge mouthful from me..

His knowledge of A fib and matters pertaining to the heart are terrible and his lack of thoroughness is a disgrace..

I live in Australia they estimate 400000 people suffer forms of A fib here which i find all the more appalling from a personal view point...

I live in the country and it can very hard here to get good help...

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This is difficult, even for me. At least I can claim to have a "science teacher" background, and "my wife is a nurse". I often get nervous about seeing doctors, simply because of the confrontation side. But, I have vivid memories, 15 years ago, of reading up on a topic, and completely changing the treatment. When I produced my evidence to a Consultant, he calmly pointed out: error of interpretation, yes-but, you have missed this, something else is more important, etc. I need both confidence, and an element of humility. I also need a good relationship. So:

1/ Learn your disease. Become knowledgeable

2/ Hold you knowledge lightly, especially in public

3/ Play the fool a bit. Respectfully ask the expert to comment on something. Or, show them something and say, "I understand it to say this, am I correct?"

4. Ask questions. Any expert who cannot explain themselves at your level to your level of satisfaction, is not demonstrating expertise.

5. Keep detailed records, and produce facts. Notice contradictions, and ask why.


Thanks and happy new year...


Good ideas re dealing with recalcitrant doctors and avoiding what I call 'medical skirmishes'. I am a retired nurse and VERY MUCH my own advocate. irina1975 'a little PR and psychology goes a long way with getting the treatment you need-not what some caregivers want you to have because it is part of their belief system.


ive been back on waf since july, i am planning on home monitoring, been pretty consistent INR only changed once, dont trust something that only gets checked every 6 months, was on eliquist had issues survived 3rd PE DONT WANT TO PUSH MY LUCK GUESSING...LOL


Hi ronnt

Were you on an anticoagulant when you had your third pulmonary embolism, and if yes was it eliquist?

Did you get a Hughes syndrome diagnosis after the tests for it that you mentioned on that forum? If you have Hughes, will you be staying on an anticoagulant for life?

My impression is that the 6 monthly testing for eliquist and the other DOACs is that it is to check kidney and liver function, not to see whether the eliquist is working. Someone will tell me if I am wrong hopefully.

Home monitoring for Warfarin adjustment needs practice to get it right. I started by doing my INR on the same day as it was done at the clinic. We then discussed whether a change in dose was necessary, and why.

It was two years before the doctor was happy to let me adjust Warfarin on my own. With your diagnosis of recurrent pulmonary embolism, the consequences of getting it wrong with home monitoring could be fatal, unlike with atrial fibrillation,, where consequences can be serious but less common.

I would think for you, that home monitoring is good to alert the clinic about out of range results, but in my opinion you should not adjust your dose of warfarin on your own. At least not without the clear agreement of your family physician. Even then, I would in your position think very hard about the risks for you of under dosing with your history of 3 pulmonary embolisms.

That said, I can see why you would prefer Warfarin over eliquist because of the tests generally available (or not ) to measure the level of control.

However I am not a doctor, all this is just the opinion of a lay person. Acting on information from the internet is often dangerous, no substitute for doctors advice when the chips are down.

But then as a Veteran of the US Military, you know that already. I take my hat off to you Sir.


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