What is a prothrombin time test with an INR (PT/INR)?

A prothrombin time (PT) test measures how many seconds it takes for a clot to form in a blood sample. Since the results of this test can vary with different labs or test methods, your results may be used to create an INR (international normalized ratio). Using this ratio makes it easier for health care providers to compare and better understand results from different labs or test methods.

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. Blood clotting is an important process that prevents you from losing too much blood when you are injured. Clotting factor levels that are too low can cause you to bleed too much after an injury. Levels that are too high can cause dangerous clots to form in your arteries or veins.

A PT/INR test helps find out if your blood is clotting normally. 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, pro time

What is it used for?

A PT/INR test is most often used to:

• See how well warfarin, an anti-coagulant or blood-thinning medicine, is working to prevent blood clots. (Coumadin is a common brand name for warfarin.) Your provider may change your dose based on these results.
• Find out the reason for abnormal blood clots.
• Find out the reason for unusual bleeding.
• Check clotting ability before a procedure or surgery that may cause bleeding.
• Check for liver problems.
• Check for vitamin K deficiency. Vitamin K helps with blood clotting.

A PT/INR test is often done along with a partial thromboplastin time (PTT) test. A PTT test also checks for clotting problems but looks at different coagulation factors than the PT test.

Why do I need a PT/INR test?

If you are regularly taking warfarin, you may need this test to make sure you are taking the right dose. Your provider will let you know how often you need to get the test to monitor your 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 darkening of the skin on the leg
• Skin that feels warm
• Trouble breathing
• Cough
• Chest pain
• Rapid heartbeat and breathing

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 have been taking warfarin for a long time, you may be trained to take your blood sample with an at-home testing kit. 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 they 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 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 or have certain health conditions, your results will probably be in the form of INR levels:

• INR levels that are too low may mean you are at risk for dangerous blood clots because your blood clots too easily.
• INR levels that are too high may mean you are at risk for dangerous bleeding because your blood is taking longer than normal to form a clot.

If your INR is not normal, your provider will probably change your dose of warfarin.

If you are not taking warfarin, your results may be in the form of a PT time, which is the number of seconds it takes for your blood sample to clot (prothrombin time).

If you are not taking warfarin and your INR or prothrombin time results are not normal, it may be a sign of that your blood is not clotting normally.

• If your PT/INR levels are too low, your blood is clotting too quickly. This may be due to:
  • A clotting disorder, a condition in which your body forms too many clots in arteries or veins. Clots can form when you don't have an injury, or they don't dissolve when they should.
  • Supplements with vitamin K, or a diet high in vitamin K.
  • Certain medicines.
• If your PT/INR levels are too high, your blood is clotting too slowly. This may be due to:
  • A bleeding disorder, a condition in which your body can't clot blood well. When you have an injury or a procedure that may cause bleeding, it may cause too much bleeding.
  • Liver disease.
  • Vitamin K deficiency. Vitamin K plays an important role in blood clotting.

Be sure to tell your provider if you are taking any medicines or supplements. Some may affect your results. Other things that may affect your PT/INR results include:

• Alcohol
• Foods high in vitamin K, such as green leafy vegetables
• Soybean and canola oils
• High-fat diets

Ask your provider what you should avoid before your PT/INR test.

To understand the results of a PT/INR test, your provider will consider your symptoms, medical history, and the results of other blood tests.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about a PT/INR test?

Sometimes certain liver tests are ordered along with a PT/INR test. These include:

• Aspartate Aminotransferase (AST)
• Alanine Aminotransferase (ALT)