PT/INR Testing

What is the PT and INR test?

The PT and INR blood test measures the time it takes your blood to form a clot. Other names for the PT test are prothrombin time and pro time. INR stands for international normalized ratio.

Why is this test done?

The PT/INR test is usually done when you are taking a blood thinner (anticoagulant) medicine, such as warfarin (Coumadin), to prevent blood clots. Clots can block blood vessels and possibly cause a heart attack or stroke. This test measures the effect of the anticoagulant. If the dosage is too high, you may bleed too easily and if it’s too low, your blood may clot too easily. The test helps your healthcare provider find the right dosage of medicine.

The PT test may also be done to diagnose a bleeding disorder if you have abnormal bleeding or clotting.

How do I prepare for this test?

  • You may need to avoid taking certain medicines before the test because they might affect the test result. Make sure your healthcare provider knows about any medicines, herbs, or supplements that you are taking. Ask your provider before stopping any of your regular medicines.
  • Talk to your healthcare provider if you have any questions about the test.

How is the test done?

Having this test will take just a few minutes. A small amount of blood is taken from a vein in your arm with a needle. The blood is collected in tubes and sent to a lab.