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.
This test may be done: when a person has a bleeding problemto monitor a person who is taking blood-thinning medicinebefore surgery to make sure a person will not bleed too much during the operation
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 After the needle is withdrawn, the puncture site is covered to prevent bleeding.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.
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.
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 warfarinis taking other medicines, such as certain antibiotics, that interfere with the testhas severe liver diseasehas disseminated intravascular coagulation (DIC), a condition usually accompanying an episode of acute critical illness, during which clotting mechanisms are abnormally activated throughout the bodyhas certain rare, inherited bleeding disordershas a vitamin K deficiency
Abnormally low PT values are usually not significant. However, they may occur when a person: has cancerhas blood clotsis taking certain medicines, such as birth control pills