Does this test have other names?
Prothrombin time/PT, Pro time, Prothrombin time/International normalized ratio, PT/INR
What is this test?
The prothrombin time is one of several tests that evaluate whether your blood is clotting
properly. Blood clotting, or coagulation, is needed to help stop bleeding. Proteins
in the blood called clotting factors or coagulants help blood become sticky and clot.
They change it from a liquid to a solid.
As soon as you get a cut or the body starts to bleed, platelet cells in the blood
collect around the bleeding area. The platelet cells and clotting factors then react
to thicken the blood and stop the bleeding. Abnormalities in the blood, including
deficiencies in clotting factors or platelets, can keep blood from clotting normally
and cause abnormal bleeding.
Clotting factors are usually made by the liver. Prothrombin is one type of clotting
factor. When bleeding occurs somewhere in the body, prothrombin quickly changes to
thrombin. The prothrombin time test measures how quickly prothrombin changes to thrombin
to stop the bleeding. If the prothrombin doesn't change as quickly as normal, a blood
clotting disorder could be the cause.
The prothrombin time test can find a lack of certain coagulants if the blood does
not clot normally. The prothrombin time test may be used to help diagnose inherited
disorders and other conditions that may affect blood clotting. These include:
Vitamin K deficiency
Deficiency in clotting factor I, II, V, VII, or X
Diseases of the liver
Problems with the bone marrow
Von Willebrand disease
Problems with the immune system
Some types of cancer, including leukemia
Why do I need this test?
Your healthcare provider may order this test if you show symptoms of having a bleeding
disorder. These symptoms can include:
Abnormal menstrual periods in women
Bleeding that occurs more easily
Bruising that occurs more easily
Blood in the stool
Your healthcare provider may also order this test regularly if you are taking a blood
thinner such as warfarin to help make sure you are getting the right dose.
You may also need this test before you have surgery or an invasive procedure to help
make sure your blood is clotting properly.
What other tests might I have along with this test?
Depending on why you are having the test, your healthcare provider may order other
blood tests along with the prothrombin time. These may include:
Thrombin time, or TT, test
Activated partial thromboplastin time, or aPTT
These tests also measure the ability of the blood to clot normally. Other tests that
measure different aspects of blood clotting, such as platelet function, may also be
What do my test results mean?
Many things may affect your lab test results. These include the method each lab uses
to do the test. Even if your test results are different from the normal value, you
may not have a problem. To learn what the results mean for you, talk with your healthcare
This test result is often reported in seconds. The normal clotting time is typically
between 10 and 12 seconds. If your blood does not clot within that normal range, you
may have a clotting or bleeding disorder.
If this test is done because you are taking an anticoagulant such as warfarin, the
result is often reported as an International Normalized Ratio, which is simply a number.
Healthcare providers usually want the INR to be between 2.0 and 3.0 for people taking
How is this test done?
The test requires a blood sample, which is drawn through a needle from a vein in your
Does this test pose any risks?
Taking a blood sample with a needle carries risks that include bleeding, infection,
bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight
stinging sensation or pain. Afterward, the site may be slightly sore.
What might affect my test results?
A number of medicines can affect the test results. These include medicines that are
used to help prevent blood clots, such as warfarin, aspirin, direct thrombin inhibitors,
factor Xa inhibitors, and heparin. Other medicines that may affect the results include:
If your blood sample is not collected correctly, your test results may also be affected.
How do I get ready for this test?
You don't usually need to prepare for this test. Your healthcare provider will tell
you whether you will need to avoid eating or drinking in the hours before the test.
Your provider may ask you to stop taking any medicines that may affect your test results,
especially those that prevent blood clotting. Be sure your provider knows about all
medicines, herbs, vitamins, and supplements you are taking. This includes medicines
that don't need a prescription and any illicit drugs you may use.