What is a prothrombin time test with an INR (PT/INR)?
A prothrombin time (PT) test measures how long it takes for a clot to form in a blood sample. An INR (international normalized ratio) is a type of calculation based on PT test results.
Prothrombin is a protein made by the liver. It is one of several substances known as clotting (coagulation) factors. When you get a cut or other injury that causes bleeding, your clotting factors work together to form a blood clot. How fast your blood clots depends on the amount of clotting factors in your blood and whether they're working correctly. If your blood clots too slowly, you may bleed too much after an injury. If your blood clots too fast, dangerous clots may form in your arteries or veins.
A PT/INR test helps diagnose the cause of bleeding or clotting disorders. It also checks to see if a medicine that prevents blood clots is working the way it should.
Other names: prothrombin time/international normalized ratio, PT protime
What is it used for?
A PT/INR test is most often used to:
- See how well warfarin is working. Warfarin is a blood-thinning medicine that's used to treat and prevent dangerous blood clots. (Coumadin is a common brand name for warfarin.)
- Find out the reason for abnormal blood clots
- Find out the reason for unusual bleeding
- Check clotting function before surgery
- Check for liver problems
A PT/INR test is often done along with a partial thromboplastin time (PTT) test. A PTT test also checks for clotting problems.
Why do I need a PT/INR test?
You may need this test if you are taking warfarin on a regular basis. The test helps make sure you are taking the right dose.
If you are not taking warfarin, you may need this test if you have symptoms of a bleeding or clotting disorder.
Symptoms of a bleeding disorder include:
- Unexplained heavy bleeding
- Bruising easily
- Unusually heavy nose bleeds
- Unusually heavy menstrual periods in women
Symptoms of a clotting disorder include:
- Leg pain or tenderness
- Leg swelling
- Redness or red streaks on the legs
- Trouble breathing
- Chest pain
- Rapid heartbeat
In addition, you may need a PT/INR test if you are scheduled for surgery. It helps make sure your blood is clotting normally, so you won't lose too much blood during the procedure.
What happens during a PT/INR test?
The test may be done on a blood sample from a vein or a fingertip.
For a blood sample from a vein:
A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.
For a blood sample from a fingertip:
A fingertip test may be done in a provider's office or in your home. If you are taking warfarin, your provider may recommend you test your blood regularly using an at-home PT/INR test kit. During this test, you or your provider will:
- Use a small needle to puncture your fingertip
- Collect a drop of blood and place it onto a test strip or other special instrument
- Place the instrument or test strip into a device that calculates the results. At-home devices are small and lightweight.
If you are using an at-home test kit, you will need to review your results with your provider. Your provider will let you know how he or she would like to receive the results.
Will I need to do anything to prepare for the test?
If you are taking warfarin, you may need to delay your daily dose until after testing. Your health care provider will let you know if there are any other special instructions to follow.
Are there any risks to the test?
There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.
What do the results mean?
If you were tested because you are taking warfarin, your results will probably be in the form of INR levels. INR levels are often used because they make it easier to compare results from different labs and different test methods. If you are not taking warfarin, your results may be in the form of INR levels or the number of seconds it takes for your blood sample to clot (prothrombin time).
If you are taking warfarin:
- INR levels that are too low may mean you are at risk for dangerous blood clots.
- INR levels that are too high may mean you are at risk for dangerous bleeding.
Your health care provider will probably change your dose of warfarin to reduce these risks.
If you are not taking warfarin and your INR or prothrombin time results were not normal, it may mean one of the following conditions:
- A bleeding disorder, a condition in which the body can't clot blood properly, causing excessive bleeding
- A clotting disorder, a condition in which the body forms excessive clots in arteries or veins
- Liver disease
- Vitamin K deficiency. Vitamin K plays an important role in blood clotting.
Learn more about laboratory tests, reference ranges, and understanding results.
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- University of Rochester Medical Center [Internet]. Rochester (NY): University of Rochester Medical Center; c2020. Health Encyclopedia: Vitamin K; [cited 2020 Jan 30]; [about 2 screens]. Available from: https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=19&contentid=VitaminK
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