The prothrombin time test (PT) helps measure how well your blood is able to clot. It is much like the partial thromboplastin time test (PTT).These two tests are often ordered together. The PT and PTT tests each measure the function of a different subset of the 12 or more proteins involved in blood clotting.
Who is a candidate for the test?
This test may be done:
- when a person has a bleeding problem
- to monitor a person who is taking blood-thinning medicine
- before surgery to make sure a person will not bleed too much during the operation
How is the test performed?
A blood sample is usually taken from a vein on the forearm or hand. The skin over the vein is first cleaned with an antiseptic. Next, a strong rubber tube called a tourniquet is wrapped around the upper arm. This restricts blood flow through the veins in the lower arm and causes them to enlarge.
A small needle is inserted into a vein and the tourniquet is removed. Blood flows from the vein through the needle. It is collected in a syringe or vial. After the needle is withdrawn, the puncture site is covered to prevent bleeding. Pressure is held on the puncture site for a bit longer than usual if a bleeding disorder, especially one involving the blood platelets, is known or suspected. The blood sample is sent to a lab for testing.
What is involved in preparation for the test?
Generally, no preparation is needed for this test. Because test preparation details may vary, a person should ask his or her healthcare professional for specific instructions.
What do the test results mean?
The healthy range for a PT test generally falls between 10 to 15 seconds, depending on technical variables unique to the particular medical lab. When a person is taking blood thinners, the international normalized ratio (INR) is calculated to determine the correct dosage of the medicine.
High PT values may occur when a person:
is taking blood-thinning medicines, especially warfarin
is taking other medicines, such as certain antibiotics, that interfere with the test
has severe liver disease
has disseminated intravascular coagulation (DIC), a condition usually accompanying an episode of acute critical illness, during which clotting mechanisms are abnormally activated throughout the body
has certain rare, inherited bleeding disorders
has a vitamin K deficiency
Abnormally low PT values are usually not significant. However, they may occur when a person:
- has cancer
- has blood clots
- is taking certain medicines, such as birth control pills