A prothrombin time (PT) test, measures the amount of time it takes for your blood plasma to clot. Prothrombin, also known as factor ll, is just one of many plasma proteins involved in the clotting process.
When you get a cut and your blood vessel is ruptured, blood platelets collect at the site of the wound, creating a temporary plug to stop the bleeding. In order to create a strong blood clot, a series of 12 plasma proteins, or coagulation “factors,” act together to create a substance called fibrin that seals the wound.
If your body does not create a certain coagulation factor or does so incorrectly, it can be due to a bleeding disorder known as hemophilia. Symptoms of a bleeding disorder include:
- easy bruising
- bleeding that won’t stop even after applying pressure to the wound
- heavy menstrual periods
- blood in the urine
- swollen or painful joints
If your doctor suspects that you may have a bleeding disorder, he or she may order a PT test to help make the diagnosis. Your doctor may also order a PT test to make sure that your blood is clotting normally before you undergo major surgery, even if you have no symptoms of a bleeding disorder.
If you are taking the blood-thinning medication warfarin, your doctor will order regular PT tests to ensure that that you are not taking so much medication that you are susceptible to excessive bleeding.
The test may also be used to check how the blood of people with liver disease or vitamin K deficiency clots. This is because these conditions can cause a bleeding disorder to develop.
Tell your doctor about all medications and supplements you are taking, and he or she will advise you whether or not to stop taking them before the test. The doctor may ask you to stop taking your blood-thinning medication because it can affect the results of the test. You will not need to fast beforehand.
You will need to have your blood drawn in order to have a PT test. Having blood drawn is an outpatient procedure, usually performed at a diagnostic lab. The procedure takes only a few minutes and is relatively painless.
A nurse will use a small needle to withdraw blood from a vein (usually in your arm or hand), and a laboratory specialist will add chemicals to the blood to see how long it takes for a clot to form.
There are very few risks associated with having your blood drawn for a PT test. However, because the test is often performed on patients who have bleeding disorders, they are at a slightly higher risk for excessive bleeding and hematoma (blood that accumulates under the skin).
You may feel slightly faint or feel some soreness or pain at the site where your blood was drawn. There is also a very slight risk of infection at the puncture site.
The normal amount of time it takes for blood plasma to clot is between 11 and 13.5 seconds if you are not taking blood-thinning medication. If you are taking this type of medication, it could take longer for a clot to form. If your blood clots within the normal amount of time, you probably do not have a bleeding disorder or you are taking the correct dose of blood-thinning medication.
If your blood does not clot in the normal amount of time, you may be on the wrong dose of warfarin or have liver disease, vitamin K deficiency, or a bleeding disorder such as Factor II deficiency. If you have a bleeding disorder, your doctor may recommend factor replacement therapy or a transfusion of blood platelets or fresh frozen plasma.
Written by: Heather Ross
Published on Jul 25, 2012
Updated on Feb 15, 2013
Medically reviewed by Brenda B. Spriggs, MD, MPH, FACP