Prothrombin time (PT) is a blood test that measures how long it takes blood to clot. A prothrombin time test can be used to check for bleeding problems. PT is also used to check whether medicine to prevent blood clots is working.
A PT test may also be called an INR test. INR (international normalized ratio) stands for a way of standardizing the results of prothrombin time tests, no matter the testing method. So your doctor can understand results in the same way even when they come from different labs and different test methods. Using the INR system, treatment with blood-thinning medicine (anticoagulant therapy) will be the same. In some labs, only the INR is reported and the PT is not reported.
Blood clotting factors are needed for blood to clot (coagulation). Prothrombin, or factor II, is one of the clotting factors made by the liver. Vitamin K is needed to make prothrombin and other clotting factors. Prothrombin time is an important test because it checks to see if five different blood clotting factors (factors I, II, V, VII, and X) are present. The prothrombin time is made longer by:
- Blood-thinning medicine, such as heparin. Another test, the activated partial thromboplastin time (APTT) test, is a better test to find out if the right dose of heparin is being used.
- Low levels of blood clotting factors.
- A change in the activity of any of the clotting factors.
- The absence of any of the clotting factors.
- Other substances, called inhibitors, that affect the clotting factors.
- An increase in the use of the clotting factors.
An abnormal prothrombin time is often caused by liver disease or injury or by treatment with blood thinners.
Another blood clotting test, called partial thromboplastin time (PTT), measures other clotting factors. Partial thromboplastin time and prothrombin time are often done at the same time to check for bleeding problems or the chance for too much bleeding in surgery.
Why It Is Done
Prothrombin time (PT) is measured to:
- Find a cause for abnormal bleeding or bruising.
- Check to see if blood-thinning medicine, such as warfarin (Coumadin), is working. If the test is done for this purpose, a PT may be done every day at first. When the correct dose of medicine is found, you will not need so many tests.
- 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 for a low level of vitamin K. Vitamin K is needed to make prothrombin and other clotting factors.
- 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
Many medicines can change the results of this test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take, as well as any supplements or herbal remedies you use.
How It Is Done
The health professional drawing blood will:
- Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
- Clean the needle site with alcohol.
- Put the needle into the vein. More than one needle stick may be needed.
- Attach a tube to the needle to fill it with blood.
- Remove the band from your arm when enough blood is collected.
- Put a gauze pad or cotton ball over the needle site as the needle is removed.
- Put pressure to the site and then a bandage.
How It Feels
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
There is very little chance of a problem from having blood sample taken from a vein.
- You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
- In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
- Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.
Prothrombin time (PT) is a blood test that measures how long it takes blood to clot.
The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.
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 blood-thinning medicine (anticoagulant therapy) will be the same. In some labs, only the INR is reported and the PT is not reported.
Prothrombin time (PT) and international normalized ratio (INR)
|Prothrombin time (PT):|
|International normalized ratio (INR):|
The warfarin (Coumadin) 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.
- 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), 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 (Coumadin) or, in rare cases, heparin.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- Taking medicines that can affect the action of blood thinners (such as warfarin) and vitamin K. These include antibiotics, aspirin, cimetidine (Tagamet), barbiturates, birth control pills, hormone replacement therapy (HRT), and vitamin K supplements.
- Having severe diarrhea or vomiting that causes fluid loss and dehydration. This may make the PT time longer. If diarrhea is caused by poor absorption of nutrients, vitamins, and minerals from the intestinal tract (malabsorption syndrome), the PT may be longer because of a lack of vitamin K.
- Eating foods that have vitamin K, such as beef liver, pork liver, green tea, broccoli, chickpeas, kale, turnip greens, and soybean products.
- Drinking a lot of alcohol.
- Taking laxatives.
- Taking some herbal products or natural remedies.
What To Think About
- The U.S. Food and Drug Administration (FDA) has approved a home test for prothrombin time (PT). If you need a PT test frequently and for a long time, you may want to ask your doctor if this home test is an option for you.
- A PT is done at the same time of day each time so test results can check whether the right dose of medicine is being used to prevent blood clots.
- Another blood clotting test, called partial thromboplastin time (PTT), measures other clotting factors. Partial thromboplastin time and prothrombin time are often done at the same time to check for bleeding problems. For more information, see the topic Partial Thromboplastin Time.
- Prothrombin levels are checked along with other liver tests, such as aspartate aminotransferase and alanine aminotransferase to check how the liver is working. For more information, see the topics Aspartate Aminotransferase (AST) and Alanine Aminotransferase (ALT).
Other Works Consulted
Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
|Primary Medical Reviewer||E. Gregory Thompson, MD - Internal Medicine|
|Specialist Medical Reviewer||Joseph O'Donnell, MD - Hematology, Oncology|
|Last Revised||September 1, 2010|