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
Why Is a Prothrombin Time Test
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 factor incorrectly, or not at all.
Some medications, liver disease, or vitamin K deficiency may cause abnormal
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 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. A PT can check how your blood clots if you have one
of these conditions.
How Is a Prothrombin Time Test
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.
What Risks Are Associated with a
Prothrombin Time Test?
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.
What Do the Test Results Mean?
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.
blood clots within the normal amount of time, you probably don’t have a
bleeding disorder. If you are taking a blood thinner type of
medication, 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.