Warfarin: tests and monitoring
To make sure that warfarin is working safely and effectively in your body, you must have regular blood tests (monitoring). It’s important you know when and how often your blood needs to be tested, and that you don’t skip any tests that your doctor advises.
What test is used to monitor warfarin?
The test used to monitor the effects of warfarin is called the International Normalised Ratio, or INR. It is a blood test that checks how long it takes for blood to clot. The higher the INR, the longer it will take blood to clot (and the higher the risk of bleeding). The lower the INR, the more likely you are to develop a blood clot.
Watch our video, in which Dr John Worthington explains what INR tests are for, the target result, and when you might need to have the test more frequently. He also talks about why being able to measure warfarin's effect with an INR test is an advantage.
Be medicinewise with warfarin
- have the INR test as often as recommended
- call your doctor or laboratory to find out your result
- keep track of your INR test results and daily warfarin dose.
Download our Living well with warfarin fact sheet — for people who’ve been prescribed warfarin, or their carers, to help you live safely with warfarin. The more you know about warfarin, the more your health will benefit.
What is the target INR?
Your INR results need to stay within a certain range. This range is called the target INR. The target INR ranges from 2 to 3 for people taking warfarin. However, some people will need to have a lower or slightly higher target INR.
If your INR is too high your risk of bleeding increases. And if your INR is too low, your blood is more likely to form a blood clot. Monitoring your INR and keeping it in the target range will help you to avoid side effects.
The target INR helps doctors to adjust your warfarin dose, which can differ from person to person, and from time to time. Your doctor will tell you what the target INR is for you.
For one person a small dose may be enough to reach the target INR, while another person may need to take twice as much to gain the same effect. It is important not to compare doses with other people — your body may respond to warfarin differently to theirs.
How often do INR tests need to be done?
When you first start taking warfarin you will need to have your INR tested frequently — maybe every 1 or 2 days for the first week — to work out your correct warfarin dose.
Once you’re stabilised on a warfarin dose that achieves your target INR, you won’t need to be tested so often. Many patients have the test once per month. From time to time you may need to have more tests — e.g. if you have any changes to your diet or medicines, as these may interact with the effect of warfarin in your body.
Because many factors can affect how warfarin works, it’s important to have your INR tests done when they are scheduled. This helps to make sure your INR stays within the target range. Talk to your doctor or pharmacist about any changes to your current medicines or diet — including when you are unwell or travelling — and before taking any new medicines, including those you buy over the counter from the pharmacy, supermarket or health food shop.
How are INR tests done?
Watch our video, in which Libby discusses INR testing for people on warfarin. Libby talks about how she has an INR test, gets the results, and finds out if she needs to adjust her dose of warfarin.
INR tests involve withdrawing blood from a vein in the arm. The blood is then sent to a laboratory to measure the INR. Your INR blood test result will go back to your doctor, who will call you to discuss your results and whether you need to change your warfarin dose, or will ask you to come in for an appointment.
Blood tests can be organised in a number of different ways. They may take place:
- at your doctor’s clinic
- at a pathology centre
- at a community pharmacy
- in a hospital clinic
- at your home, by arrangement (e.g. with the pathology service).
Portable monitoring devices
Besides laboratory testing, a portable device can be used to test your INR. Some healthcare professionals (nurses, doctors or pharmacist) will do this type of testing on the spot. This is called ‘point of care’ testing.
The device used is very similar to those used to test blood sugar levels in people with diabetes. Only a drop of blood is needed to do the test, which is obtained from a finger prick. The device gives the results within a few minutes.
Some people may be able to do this testing at home after they’ve received thorough training — this is called ‘self-monitoring’. In order to do self-monitoring, you will need to have your tests as often as your doctor tells you to, and be diligent with following the special lifestyle instructions that are advised when taking warfarin. Most people will still need to contact your doctor to report your INR and to check if your warfarin dose needs to be changed.
Speak to your doctor to work out the best monitoring option for you.
Note: Portable monitoring devices and the replacement strips are not covered by Medicare — you would need to cover the cost yourself. If you have private health insurance, your plan may cover it. You can check this with your insurance provider.
- Good Health Care website. INR monitoring: patient self-monitoring. Sydney: Setform Ltd, 2010. www.goodhealthcare.com.au/inr-monitoring/patient-self-monitoring (accessed 24 April 2012).
- Unit for Medication Outcomes Research and Education (UMORE). Anticoagulation website: self-monitoring options. Tasmania: University of Tasmania, 2009. www.anticoagulation.com.au/SelfMonitoring/Options/tabid/84/Default.aspx (accessed 24 April 2012).