Patient education: Prothrombin time and INR (PT/INR) (The Basics)

What is prothrombin time and INR? — A prothrombin time test is a blood test that measures how long it takes the blood to clot. It is also called a "PT" or "pro time" test.

Different labs use different tests to measure PT. Because of this, a different number is used to report the results. This is 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 or seen in a different office).

Your PT and INR tell your doctor if your blood is clotting at the normal speed.

Why do I need a PT/INR test? — You need regular PT/INR tests if you take warfarin (sample brand name: Jantoven). This is an "anticoagulant" medicine used to help prevent blood clots. Doctors prescribe it to people who might get blood clots or who have had a blood clot in the past. When you are taking warfarin, your INR is increased. Your doctor or pharmacist will use the INR number to adjust your warfarin dose.

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

●You are having surgery (but not everyone having surgery needs this test).

●You have bleeding symptoms, such as heavy menstrual periods, nosebleeds, bleeding gums, or blood in your urine or bowel movements.

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

For these other reasons for getting a PT/INR, you will probably have other tests to check blood clotting as well. The PT/INR is only 1 way to test blood clotting.

What should my INR be? — For a normal PT (meaning that your blood is clotting at the normal speed), the INR is around 1. If you are not taking warfarin or certain other "anticoagulant" medicines and your INR is increased, your doctor might do tests to understand why.

The INR for someone taking warfarin is usually between 2 and 3. In some cases, the doctor might try to keep it very close to 2.5. Make sure that you know what your INR should be.

If I take warfarin, how often do I need a PT/INR test? — You might get a PT/INR test before you begin taking warfarin. But some people only get the test after starting warfarin.

After starting warfarin:

●You will probably get a PT/INR test every day if you are in the hospital, or after 2 or 3 doses of warfarin (2 or 3 days) if you are not in the hospital.

●Your doctor, pharmacist, or anticoagulation clinic will then adjust your warfarin dose. They will recheck the INR until it is where it should be.

●You might need PT/INR tests every few days for a while, until the dose of warfarin is right. Once your test result is good for at least 1 to 2 weeks, you can probably get the test once a month.

You might have a PT/INR test more often if something changes, like 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 use a small monitor to test their PT/INR at home. This is called "self-monitoring." Talk to your doctor, nurse, or pharmacist about whether this is an option for you.

If you are doing self-monitoring:

●You will use a special needle to prick your finger to get a drop of blood. Clean the finger first to prevent infection.

●Each machine has a different way to test the drop of blood. Many of them use a test strip or chip that goes into the machine.

●The machine shows your PT/INR value.

●You let your doctor, nurse, or pharmacist know your INR result. They will tell you if you should change your warfarin dose or keep taking the same dose as before.

●Some people can learn how to adjust their own warfarin dose. Only do this if you have been taught how to do it and have the OK from your doctor, nurse, or pharmacist.

More on this topic

Patient education: Choosing an oral medicine for blood clots (The Basics)
Patient education: Taking oral medicines for blood clots (The Basics)
Patient education: Warfarin and your diet (The Basics)

Patient education: Warfarin (Beyond the Basics)