A prothrombin time (PT) test measures the amount of time it takes for your blood plasma to clot. Prothrombin, also known as factor II, is just one of many plasma proteins involved in the clotting process.
When you get a cut and your blood vessel ruptures, blood platelets collect at the site of the wound. They create a temporary plug to stop the bleeding. In order to produce a strong blood clot, a series of 12 plasma proteins, or coagulation “factors,” act together to make a substance called fibrin, which seals the wound.
A bleeding disorder known as hemophilia could cause your body to create certain coagulation factors incorrectly, or not at all. Some medications, liver disease, or vitamin K deficiency may also cause abnormal clot formation.
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 you have a bleeding disorder, they may order a PT test to help them make a diagnosis. Even if you have no symptoms of a bleeding disorder, your doctor may order a PT test to make sure your blood is clotting normally before you undergo major surgery.
If you’re taking the blood-thinning medication warfarin, your doctor will order regular PT tests to ensure you’re not taking too much medication. Taking too much warfarin can cause excessive bleeding.
Liver disease or vitamin K deficiency can cause a bleeding disorder. Your doctor may order a PT to check how your blood clots if you have one of these conditions.
Blood-thinning medication can affect the results of the test. Tell your doctor about all medications and supplements you’re taking. They’ll advise you whether to stop taking them before the test. You will not need to fast before a PT.
You’ll need to have your blood drawn for a PT test. This is an outpatient procedure usually performed at a diagnostic lab. It takes only a few minutes and causes little to no pain.
A nurse or phlebotomist (a person specially trained in drawing blood) will use a small needle to draw blood from a vein, usually in your arm or hand. A laboratory specialist will add chemicals to the blood to see how long it takes for a clot to form.
Very few risks are associated with having your blood drawn for a PT test. However, if you have a bleeding disorder, you’re at a slightly higher risk for excessive bleeding and hematoma (blood that accumulates under the skin).
There’s a very small risk of infection at the puncture site. You may feel slightly faint or feel some soreness or pain at the site where your blood was drawn. You should alert the person administering the test if you begin to feel dizzy or faint.
Blood plasma normally takes between 11 and 13.5 seconds to clot if you’re not taking blood-thinning medication. PT results often are reported as an international normalized ratio (INR) that’s expressed as a number. A typical range for a person not taking blood thinner medication is 0.9 to about 1.1. For someone taking warfarin, the planned INR is usually between 2 and 3.5.
If your blood clots within the normal amount of time, you probably don’t have a bleeding disorder. If you are taking a blood thinner, a clot will take longer to form. Your doctor will determine your goal clotting time.
If your blood doesn’t clot in the normal amount of time, you may:
- be on the wrong dose of warfarin
- have liver disease
- have vitamin K deficiency
- have 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.