Prothrombin Time and INR
Prothrombin time (PT) is a blood test that measures how long it takes blood to clot. A prothrombin time test can be used to check for bleeding problems. PT is also used to check whether medicine to prevent blood clots is working.
A PT test may also be called an INR test. INR (international normalized ratio) stands for a way of standardizing the results of prothrombin time tests, no matter the testing method. So your doctor can understand results in the same way even when they come from different labs and different test methods. Using the INR system, treatment with blood-thinning medicine (anticoagulant therapy) will be the same. In some labs, only the INR is reported and the PT is not reported.
Blood clotting factors are needed for blood to clot (coagulation). Prothrombin, or factor II, is one of the clotting factors made by the liver. Vitamin K is needed to make prothrombin and other clotting factors. Prothrombin time is an important test because it checks to see if five different blood clotting factors (factors I, II, V, VII, and X) are present. The prothrombin time is made longer by:
An abnormal prothrombin time is often caused by liver disease or injury or by treatment with blood thinners.
Another blood clotting test, called partial thromboplastin time (PTT), might be used if you take another type of blood-thinning medicine called heparin. This test measures other clotting factors. Partial thromboplastin time and prothrombin time are often done at the same time to check for bleeding problems or the chance for too much bleeding in surgery.
Why It Is Done
Prothrombin time (PT) is measured to:
How To Prepare
Many medicines can change the results of this test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take, as well as any supplements or herbal remedies you use.
How It Is Done
The health professional drawing blood will:
In some cases the health professional will take a sample of blood from your fingertip instead of your vein. For a finger stick blood test, the health professional will clean your hand, use a lancet to puncture the skin, and place a small tube on the puncture site in order to collect your blood.
How It Feels
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
There is very little chance of a problem from having a blood sample taken from a vein.
Prothrombin time (PT) is a blood test that measures how long it takes blood to clot.
The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.
A method of standardizing prothrombin time results, called the international normalized ratio (INR) system, has been developed so the results among labs using different test methods can be understood in the same way. Using the INR system, treatment with blood-thinning medicine (anticoagulant therapy) will be the same. In some labs, only the INR is reported and the PT is not reported.
The warfarin (Coumadin) dose is changed so that the prothrombin time is longer than normal (by about 1.5 to 2.5 times the normal value or INR values 2 to 3). Prothrombin times are also kept at longer times for people with artificial heart valves, because these valves have a high chance of causing clots to form.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
What To Think About
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