Prothrombin Time (PT/INR) Test

Why It Is Done

Prothrombin time (PT) is measured to:

• Find a cause for abnormal bleeding or bruising.
• Check the effects of warfarin (Coumadin). You will have the test regularly to make sure you are taking the right dose.
• Check for low levels of blood clotting factors. The lack of some clotting factors can cause bleeding disorders such as hemophilia, which is passed in families (inherited).
• Check if it is safe to do a procedure or surgery that might cause bleeding.
• Check how well the liver is working. Prothrombin levels are checked along with other liver tests, such as aspartate aminotransferase and alanine aminotransferase.
• Check to see if the body is using up its clotting factors so quickly that the blood can't clot and bleeding does not stop. This may mean the person has disseminated intravascular coagulation (DIC).

How To Prepare

In general, there's nothing you have to do before this test, unless your doctor tells you to.

How It Is Done

A health professional uses a needle to take a blood sample, usually from the arm.

In some cases, the health professional will take a sample of blood from your fingertip instead of your vein. For a finger stick blood test, the health professional will clean your hand, use a lancet to puncture the skin, and place a small tube on the puncture site to collect your blood.

Some people use a monitor at home to test a small blood sample.

How long the test takes

The test will take a few minutes.

How It Feels

When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.

Risks

There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.

Results

Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.

A method of standardizing prothrombin time results, called the international normalized ratio (INR) system, has been developed so the results among labs using different test methods can be understood in the same way. Using the INR system, treatment with warfarin (Coumadin) will be the same. In some labs, only the INR is reported and the PT is not reported.

The warfarin dose is changed so that the prothrombin time is longer than normal (by about 1.5 to 2.5 times the normal value or INR values 2 to 3). Prothrombin times are also kept at longer times for people with artificial heart valves, because these valves have a high chance of causing clots to form.

Abnormal values

• A longer-than-normal PT can mean a lack of or low level of one or more blood clotting factors (factors I, II, V, VII, or X). It can also mean a lack of vitamin K; liver disease, such as cirrhosis; or that a liver injury has occurred. A longer-than-normal PT can also mean that you have disseminated intravascular coagulation (DIC). This is a life-threatening condition in which your body uses up its clotting factors so quickly that the blood cannot clot and bleeding does not stop.
• A longer-than-normal PT can be caused by treatment with blood-thinning medicines, such as warfarin or, in rare cases, heparin.