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A prothrombin time test (also called a PT or “pro time” test) is a blood test that measures how long it takes your blood to clot.

Different labs use different tests to measure PT. Because of this, the PT is changed to a standard number, called the International Normalized Ratio (INR). This number can be used for comparison even if you get the test done in different labs (if you are traveling, for example). Your PT and INR tell your doctor if your blood is clotting too much or too little.

Why do I need a PT/INR test?You need regular PT/INR tests if you take warfarin (sample brand name: Jantoven). This medicine helps prevent blood clots. Doctors prescribe it to people who might get blood clots or who have had a blood clot in the past.

If you are not taking warfarin, your doctor might do a PT/INR test if:

You are going to have surgery and they are concerned about unexpected bleeding

You have symptoms of a bleeding problem, such as nosebleeds, bleeding gums, or blood in your urine or bowel movements

You have very heavy menstrual periods

You have a condition that could affect how your blood clots, such as liver disease

If I take warfarin, how often do I need a PT/INR test? You will get a PT/INR test before you begin taking warfarin, and then again after about 3 doses of warfarin.

Your doctor will then adjust your warfarin dose to get your PT/INR to the right level. For most people, that means an INR between 2 and 3. You might need PT/INR tests every few days for a while, to make sure you’re taking the right dose of warfarin. Once your test result stays the same for at least 1 to 2 weeks, you can probably get tests just once a month. You might have a PT/INR test more often if something changes, such as if you get sick, start a new medicine that affects your PT/INR, or have any bleeding.

Can I do a PT/INR test at home? Some people are able to use a home monitor to test their PT/INR at home. Talk to your doctor, nurse, or pharmacist about whether you could use a home monitor.

PT home testing machines are about the size of a small book. You use the machine to test a drop of blood from your finger. The readout on the machine shows your PT/INR value.

You then report the number to your doctor, nurse, or pharmacist. They will tell you if you should change your warfarin dose. This is called “self-monitoring.”

Some people might be able to learn how to adjust their own warfarin dose. This is called “self-management.” Self-management is only for people who have been trained and have shown that they know how to change their dose. Do not try self-management without the OK of your doctor, nurse, or pharmacist.