Version July 2025
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What is prothrombin time and INR? —
A prothrombin time test ("PT test" or "pro time test") is a blood test to measure how long it takes the blood to clot.
Different labs use different tests to measure PT. Because of this, the PT from 1 lab cannot be compared with that of another lab. To get around this problem, a different number is used to report the results. This is called the "international normalized ratio" ("INR"). This is calculated based on the PT and can be used for comparison even if you get the test done in different labs. For example, this might be the case 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 or treat blood clots. Doctors prescribe it to people who had a blood clot or might get them.
Taking warfarin raises your INR because it partially blocks blood clotting. 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:
●Will have surgery (but not everyone having surgery needs this test)
●Have bleeding symptoms, such as heavy menstrual periods, nosebleeds, bleeding gums, or blood in urine or bowel movements
●Have a condition that could affect how your blood clots, such as liver disease
If you do not take warfarin, you will probably have other tests to check blood clotting as well. The PT/INR is only 1 way to test it.
What should my INR be? —
For a normal PT (meaning your blood is clotting normally), 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 figure out 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 you know what your INR should be.
If I take warfarin, how often do I need a PT/INR test? —
You might get the 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 the 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 use a test strip or chip that goes into the machine.
●The machine shows your PT/INR result.
●Tell your doctor, nurse, or pharmacist the result. They will tell you if you should change your warfarin dose.
●Some people can learn how to adjust their own warfarin dose. Only do this if you have been taught how and have the OK from your doctor, nurse, or pharmacist.
Patient education: Anticoagulant medicines – Uses and kinds (The Basics)
Patient education: How to take anticoagulants safely (The Basics)
Patient education: Warfarin and your diet (The Basics)
Patient education: Partial thromboplastin time test (The Basics)
Patient education: Clotting factor blood test (The Basics)
