INR and self-testing

Most APS patients will be on life-long anticoagulation and, for those who have had a serious clotting event, the current treatment usually takes the form of warfarin.


Warfarin is a powerful anticoagulant that interferes with your blood clotting process in order to prevent blood clots from forming. It has been used as a medicine since 1954 and is considered to be a fairly safe and stable drug.

One major drawback of warfarin treatment is that, because of the way it affects your blood clotting process, your blood needs to be regularly monitored. This is not the case with other anticoagulants such as aspirin, heparin and rivaroxaban as they work in a different way.

It is necessary to measure your blood thickness because it must fall within the INR range specified by your doctor: if it is too high, there is a risk of bleeding, but if it is too low there is a danger of clotting.

The most common side effects of warfarin are bleeding and bruising such as prolonged bleeding from small cuts, nosebleeds, large bruises under the skin, and bleeding gums when brushing your teeth. You should seek medical advice if you notice any sign of major bleeding or bruising.


INR is an acronym for the International Normalised Ratio and is a test which measures the length of time it takes your blood to clot compared to normal; normal blood has an INR of approximately 1.0. The INR test was developed by the World Health Organisation so that tests would be standard throughout the world, allowing people who have to take life-long warfarin to travel and get comparable blood tests wherever they are.

INR range

Each patient has a target INR which is set by a consultant according to their condition or disease and medical history. The dosage of warfarin can vary widely from person to person, some patients only need 4mg a day while others may need 18mg or more; therefore, when you first start treatment, the warfarin dose will be adjusted up or down to achieve your target INR. The strength of warfarin tablets is clearly marked by the colour and number stamped on them. If the INR is too low then the dosage is increased; if the INR is too high then the dosage is decreased.

For the majority of people who have heart conditions such as atrial fibrillation or who have had a blood clot but do not have APS, an INR of between 2.0-3.0 is usually sufficient and tends to be the norm.

However, in a number of patients with APS, the blood is extremely prone to clotting and an INR of 3.0-4.5 is sometimes necessary. The aim is to find an INR target range as low as possible, but for the patient to be relatively symptom-free. This message is very important as patients who continue to live with symptoms can find the condition quite debilitating or may even have further severe clotting episodes.

INR monitoring

Everyone taking warfarin will have their INR monitored through regular blood tests either by a venous sample or finger prick test carried out in their GP's surgery or anticoagulation clinic.

It may take several weeks for your target INR to be reached mainly due to the fact that warfarin reacts with many external factors which, in turn, can affect your INR. Warfarin is particularly sensitive to diet, alcohol and other medication – please look at our Living with APS section, for detailed information, and we also recommend that you watch the video produced by the anticoagulation unit at Southampton Hospital which gives practical advice and information about warfarin.

Your INR will probably be checked daily until it does reach your therapeutic range. After that it will be about twice a week for the next one to two weeks, then weekly until it is stable; once it stabilises, you should then have tests every six to twelve weeks. If you do need new medication or you become ill, it will be necessary to be tested more frequently until the INR settles down again.

However, many APS patients will find that their INR fluctuates for reasons still unknown and it rarely stabilises. Consequently, a lot of people have benefitted from self-testing their own INR.

Patient self-testing

Self-monitoring can be particularly useful for people with busy lives who have work/family commitments, for those who find it difficult to travel to clinics due to distance or disabilities, for overseas travellers and, of course, people who have highly unstable INRs.

You can self-test by using a portable hand-held machine to measure the INR in a drop of blood. This gives you freedom to test from the comfort of your own home, at work or while away on holiday or business.

There are several monitors available on the international market but, in the UK, there are currently only two: the Coaguchek XS which is made by Roche Diagnostics and INRatio which is manufactured by Alere.

When you self test, you take your own INR reading from the monitor and then give this data to your doctor or nurse. Based on this information, they will then advise you on the dose of warfarin you need to take. Following appropriate training and experience, some people feel confident enough to self-manage. This means they take the INR reading and adjust the dosage themselves.

Points to consider before buying a self-testing machine

Self-monitoring is not for everyone, but it can give you a better quality of life and lets you play an active role in your own health care.

However, before you make the decision to buy a monitor, please consider the following points:

  • You must be manually dexterous so you can operate the machine. It involves taking a finger-prick test and applying it to a test strip.
  • You should have reasonable eyesight so you can take the readings as well as use the monitor.
  • Can you get the test strips on prescription? If your GP’s surgery is unable to prescribe the strips they cost approximately £85 for a box of 24 (based on 2016 prices) – this is unaffordable for most people.
  • Will you have the full support of your GP or anticoagulation clinic? This is essential as they will help to train you to use the monitor, be prepared to take your readings and decide what to do with the results. We suggest you make an appointment at your GP surgery/clinic to discuss your options.
  • Does your budget stretch to £299? Both monitors on the UK market currently retail for £299 and they are not available on the NHS.
  • You should still be prepared to make regular visits to the surgery/clinic initially so that the readings you take from your monitor can be compared to readings obtained at the clinic.

Will the INR results be the same?

It is now widely accepted that finger-prick testing is just as accurate as venous tests with many surgeries now opting for finger-prick testing as standard.

However, it is common for the INR results between the two to differ slightly:

  • With an INR range up to 2.5 you can expect a difference of between 0.1- 0.3
  • With an INR range of between 2.5-4.5 you can expect a difference of between 0.5-1.0
  • With an INR of more than 4.5 you can expect a difference of between 1.0—2.0

This small discrepancy is caused by the different sampling methods and is considered clinically acceptable as long as it is consistent. Therefore, it important to run parallel testing for several weeks in which your own monitor results are compared to those obtained in the anticoagulation clinic venous tests, and that your healthcare team is aware of this difference.

From patient feedback, we also advise that you recheck this differential each time you use a new batch of testing strips as it can vary slightly. The testing strips are supplied in batches of 24 or 48 so, when a new one is opened, make sure you get a venous reading from the anticoagulation clinic, compare this with your monitor’s results and make a note of the difference.

Is it safe for APS patients to self-test?

Due to the instability of INR levels in APS patients, self-testing is often a sensible approach both in terms of looking after your health and managing your quality of life. It allows you to test immediately if you are feeling unwell, and also means you can get on with your life with fewer visits to the anticoagulation clinic.

Once the slight consistent discrepancy discussed above has been identified, it will be possible to know exactly what your INR range should be on the self-testing monitor. With this taken into consideration, self-testing for APS patients is considered to be as suitable as for anyone else but there are special precautions for people who test positive for the lupus anticoagulant.

There are three tests which look for antiphospholipid antibodies: the anticardiolipin (aCL), the anti-beta2-glycoprotein1 (anti-B2GP1) and the lupus anticoagulant (LA). The LA has been found to interfere with the reagents which are used in the self-testing strips and, consequently, given false high readings in a very small number of cases (4.3%).

Both Roche Diagnostics who make the Coaguchek XS and Alere who produce INRatio state that testing may be unsuitable for some people who are positive for the LA. Roche Diagnostics issued the following statement in 2010:

There is a warning on the pack inserts for the Coaguchek XS testing strips which states:

“Antiphospholipid antibodies (APA) such as the Lupus Anticoagulant (LA) may falsely prolong coagulation times ie. they may cause false high INR values and false low values. Where APA are known to be present, it is imperative that a result be obtained using an APA-insensitive laboratory method for comparison”.

Roche still recommends that the Coaguchek XS is suitable for patients with antiphospholipid antibodies as long as they have existing specialist advice, and understanding and support of their specialist. If a patient has a positive LA result, then Roche recommend that they continue having venous tests alongside the Coaguchek XS tests for three months to make sure the readings are stable.

What to do if you are positive for the Lupus Anticoagulant

First of all find out whether you tested positive for the lupus anticoagulant – you will be able to find this information from either your GP or consultant.

If you are LA positive, then you need to point this out to your doctor or anticoagulation clinic when you have the appointment to discuss the possibility of you self-testing with their support.

GPs and anticoagulation clinics often run parallel venous testing alongside your own finger-prick readings in order to establish what the differential is, then draw up an individual management plan based on the comparative INR results. If you are LA positive you must run these parallel tests for three months.

Taking the usual small discrepancy into account, if the results are comparable then it would be safe to self-test. If they are wildly different, then self-testing would not be advised.

How do I start self monitoring?

Go through the list of points to consider before buying a self-testing machine.

If you think you meet the criteria and it would improve your quality of life, make an appointment and discuss this with the medical professional who manages your anticoagulation.

  • If they are willing to supervise your training and draw up an INR management plan, you should look into buying the self-testing machine.
  • Plan and implement your training sessions with your supervisor. As well as receiving training and education from your GP or anticoagulation clinic, there is a webpage provided by Roche Diagnostics that can help teach you how to use the CoaguChek XS machine yourself.
  • Once you have received the training, and your anticoagulation professional is satisfied that you are capable and comfortable performing the tests yourself and recording them correctly, you should be asked to sign a written contract setting out your individual management plan.

INR and self testing fact sheet

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