How can you monitor and adjust treatment with vitamin K antagonists?

Anticoagulant (blood-thinning) medications like vitamin K antagonists are used to prevent blood clots. This can lower the risk of certain cardiovascular (heart and blood vessel) diseases, especially strokes. The medication is often prescribed for people who have an irregular heartbeat like atrial fibrillation or flutter, and for people who have an artificial heart valve. Vitamin K antagonists are also used by people who have deep vein thrombosis or pulmonary (lung) embolism – to treat it, and sometimes to prevent further blood clots as well.

Vitamin K antagonists are sometimes called coumarins. The most commonly used vitamin K antagonist in Germany is phenprocoumon. It is best known under the trade name Marcumar, but also as Falithrom and Phenprogamma. Warfarin is the most widely prescribed anticoagulant in some other countries.

These medications slow down the blood-clotting process. The clotting ability of the blood has to be monitored regularly in order to adjust the dose: It should be high enough to prevent blood clots – but low enough to minimize the risk of serious bleeding.

How can you measure and adjust the clotting ability of blood?

People who are on vitamin K antagonists usually have the clotting ability of their blood checked regularly. This is done by taking a blood sample and then measuring the INR (international normalized ratio). The INR reflects the blood-clotting speed.

An INR around 1 is considered to be normal in people who aren’t taking anticoagulant medication. This value is higher in people who are taking anticoagulant medication because it takes longer for their blood to clot. An INR of 2, for example, means that the blood takes twice as long to clot. Different people may have different target INR values, mainly depending on their medical condition.

The blood's clotting ability is first measured on the third day of taking the medication. That's how long it takes for the medication to start working. This blood test is done a few times per week at first. If the INR hardly changes over several consecutive measurements, the blood tests can be done less frequently. If the INR stays within the target range over several months, it’s enough to then only check the values every four weeks.

It is important to check the clotting ability of your blood more often if you

  • make big changes to your diet or lifestyle,
  • develop other illnesses, or
  • start or stop taking other medications.

In people with or a pulmonary embolism, the target INR range is between 2 and 3. If the measured value is higher (e.g. 3.5), the medicine dose is reduced. If it’s lower (e.g. 1.5), the dose is increased a bit. Because the medication doesn't work immediately, it generally takes a few days for the INR value to change.

How can you do blood tests and adjust medication yourself?

You don't always have to go to the doctor to have your blood tested. Instead, you can attend a patient education class to learn how to measure your INR yourself. You are also shown how to adjust your dose yourself. People who monitor their blood's clotting ability themselves only have to go to the doctor's every few months.

To monitor your INR yourself, you prick your finger and smear a drop of blood onto a test strip. The test strip is then inserted into a special device that measures the INR value.

Many people who take anticoagulants prefer to self-monitor because it means that they're more flexible and have fewer check-ups at the doctor's. Studies have also shown that people who monitor their own blood are less likely to develop blood clots and resulting complications.

Some people are hesitant to monitor their own blood. Others aren't able to, for instance because they have poor vision. Then family members or close friends can help them.

What can I learn in classes, and where?

The patient education doesn't only teach you how to monitor your own blood and adjust your medication. It also provides important information about many other aspects of treatment, like interactions with other medications, how your diet can influence your blood's clotting speed, and what to bear in mind if you're going to have an operation and are taking anticoagulants. There are also special classes for loved ones who would like to help.

In Germany, statutory health insurers usually cover the costs of patient education if it’s prescribed by a doctor. It is a good idea to contact your health insurer and check beforehand. The classes are offered in places like doctor's practices and hospitals, for instance.

IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.

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Please note that we do not provide individual advice on matters of health. You can read about where to find help and support in Germany in our information “How can I find self-help groups and information centers?

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Updated on October 25, 2022

Next planned update: 2025


Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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