If you take a blood-thinning medication such as warfarin to prevent clots from forming, your doctor will recommend a prothrombin time test to monitor the medication's effectiveness. It can also be ordered to:
- Detect a bleeding disorder
- Diagnose liver problems
- Screen people having surgery for unrecognized bleeding problems
A variation of the prothrombin time test is part of a test series called model for end-stage liver disease (MELD) used to assess the severity and priority of people waiting for liver transplants.
If your doctor suspects you may have liver problems, you may undergo additional testing to assess the health of your liver, such as liver enzyme tests. If your doctor suspects you may have a bleeding disorder, you may undergo additional clotting-function tests.
A prothrombin time test is safe. Because the test involves drawing blood from a vein in your arm, you may experience soreness or bruising at the site from which your blood is drawn.
During the test
Prothrombin time testing is done using a blood sample. Usually, the blood is drawn through a small needle inserted into a vein in the bend of your arm. You'll be asked to roll up your shirt sleeve if you're wearing long sleeves. The person drawing the blood might tie a band around your upper arm and ask you to make a fist. This causes your vein to stick out more, making it easier to insert the needle. The needle is attached to a small tube, in which your blood is collected. All of this usually takes just a few minutes.
You may feel a quick pain as the needle is inserted and experience some short-term discomfort at the site after the needle is removed.
After the test
Once the needle is removed, you'll be given a piece of gauze and a bandage to cover the area where the needle was inserted. You'll be asked to gently apply pressure to the area for a minute or so, to help stop any bleeding.
Your blood will be sent to a laboratory for analysis. If the laboratory analysis is done on-site, you could have your test results within hours. If your doctor sends your blood to an off-site laboratory, it may take several days to receive the results.
Home testing kits are available for people who have to take blood thinners for long periods and who have been trained in taking blood samples and testing them.
Prothrombin time test results can be presented in two ways:
- Prothrombin time in seconds. Prothrombin time is usually measured in seconds — the time it takes for your blood to clot. This way of determining prothrombin time creates results that will vary depending on the laboratory and the method used to test the blood, but a sample range is approximately 10 to 14 seconds. A number higher than average means it takes blood longer than usual to clot. A lower number means blood clots more quickly than expected.
- Prothrombin time as a ratio. For people taking the blood-thinning medication warfarin (Coumadin), results are given as a number that represents a ratio called the international normalized ratio (INR). The INR is a formula that adjusts for differences in the chemicals used in different laboratories so that test results can be comparable.
An INR range of 2.0 to 3.0 is generally effective for people taking warfarin who need full anticoagulation, but may need to be slightly higher in certain situations. If your INR is higher than this range, that means your blood clots more slowly than desired. A lower INR means your blood clots more quickly than desired. The INR is used only for people on oral anticoagulant therapy. It's not useful in people whose PT is higher for other reasons.
What your results mean
Clotting too slowly
If your prothrombin time test reveals that your blood is clotting too slowly, this can be caused by:
- Blood-thinning medications, such as warfarin (Coumadin), heparin
- Liver problems
- Inadequate levels of proteins (factors) that cause blood to clot
- Vitamin K deficiency
- Congenital factor deficiency
- Presence of coagulation factor inhibitors
Clotting too fast
If your prothrombin time test reveals that your blood is clotting too fast, this can be caused by:
- Supplements that contain vitamin K
- High intake of foods that contain vitamin K, such as liver, broccoli, chickpeas, green tea, kale, turnip greens and products that contain soybeans
- Estrogen-containing medications, such as birth control pills and hormone replacement therapy