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.
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:
- Blood-thinning medicine, such as heparin.
Another test, the activated partial thromboplastin time (APTT) test, is a
better test to find out if the right dose of heparin is being
- Low levels of blood clotting factors.
- A change in
the activity of any of the clotting factors.
- The absence of any of
the clotting factors.
- Other substances, called inhibitors, that
affect the clotting factors.
- An increase in the use of the clotting factors.
An abnormal prothrombin time is often caused by liver
disease or injury or by treatment with blood thinners.
blood clotting test, called partial thromboplastin time (PTT), 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
- Find a cause for abnormal bleeding or
- Check to see if blood-thinning medicine, such as warfarin
(Coumadin), is working. If the test is done for this purpose, a PT may be done
every day at first. When the correct dose of medicine is found, you will not
need so many tests.
- Check for low levels of blood clotting factors.
The lack of some clotting factors can cause bleeding disorders such as
hemophilia, which is passed in families
- Check for a low level of vitamin K. Vitamin K is
needed to make prothrombin and other clotting factors.
- Check how
well the liver is working. Prothrombin levels are checked along with other
liver tests, such as aspartate aminotransferase and alanine
- Check to see if the body is using up its clotting
factors so quickly that the blood can't clot and bleeding does not stop. This
may mean the person has
disseminated intravascular coagulation (DIC).