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Prothrombin Time and International Normalized Ratio
Este artigo foi revisto pela última vez em
Este artigo foi modificado pela última vez em
24 de Maio de 2018.
At a Glance
Why Get Tested?
To check how well blood-thinning medications (anti-coagulants) are working to prevent blood clots; to help detect and diagnose a bleeding disorder
When To Get Tested?
If you are taking an anti-coagulant drug or if your doctor suspects that you may have a bleeding disorder
Sample Required?
A blood sample drawn from a vein in the arm; sometimes blood from a fingerstick
Test Preparation Needed?
None needed, although if you are receiving anticoagulant therapy, the specimen should be collected before taking your daily dose.
What is being tested?
The prothrombin time (PT) test measures how long it takes for a clot to form in a sample of blood. In the body, the clotting process involves a series of sequential chemical reactions called the coagulation cascade, in which coagulation or “clotting” factors are activated one after another and result in the formation of a clot. Prothrombin is one of the coagulation factors produced by the liver. One of the final steps of the cascade is the conversion of prothrombin (factor II) to thrombin. The PT test evaluates the integrated function of the coagulation factors that comprise the extrinsic and common pathways of the coagulation cascade, including factors I (fibrinogen), II (Prothrombin), V, VII and X. It evaluates the body’s ability to produce a clot in a reasonable amount of time and, if any of these factors are deficient, the PT will be prolonged.

The PT test is usually measured in seconds and is compared to values in healthy individuals. Because the reagents used to perform the PT test vary from one laboratory to another and even within the same laboratory over time, the normal values also will fluctuate. To standardize results across the U.S. and the world, a World Health Organization (WHO) committee developed and recommended the use of the Internationalized Normalized Ratio (INR) with the PT test for patients who are receiving the blood-thinning medication warfarin (Coumadin). The INR is a calculation that adjusts for changes in the PT reagents and allows for results from different laboratories to be compared. Most laboratories are now reporting both PT and INR values whenever a PT test is performed. However, the INR should not be used to evaluate PT for patients who are not on blood-thinning medications.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm or, sometimes, from a fingerstick.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed. If the patient is receiving anticoagulant therapy, the specimen should be collected before he takes his daily dose.
Accordion Title
Common Questions
  • How is it used?

    Since the Prothrombin time (PT) evaluates the ability of blood to clot properly, it can be used to help diagnose bleeding. When used in this instance, it is often used in conjunction with the PTT to evaluate the function of all coagulation factors. Occasionally, the test may be used to screen patients for any previously undetected bleeding problems prior to surgical procedures.

    The International Normalized Ratio (INR) is used to monitor the effectiveness of blood thinning drugs such as warfarin (Coumadin). These anti-coagulant drugs help inhibit the formation of blood clots. They are prescribed on a long-term basis to patients who have experienced recurrent inappropriate blood clotting. This includes those who have had heart attacks, strokes, and deep vein thrombosis (DVT). Anti-coagulant therapy may also be given as a preventative measure in patients who have artificial heart valves and on a short-term basis to patients who have had surgeries, such as knee replacements. The anti-coagulant drugs must be carefully monitored to maintain a balance between preventing clots and causing excessive bleeding.

  • When is it ordered?

    If you are taking an anti-coagulant drug, your doctor will check your PT/INR regularly to make sure that your prescription is working properly and that your PT/INR is appropriately prolonged. There is no set frequency for doing the test. Your doctor will order them often enough to make sure that the drug is producing the desired effect - that it is increasing your clotting time to a therapeutic level without causing excessive bleeding or bruising.

    The PT may be ordered when a patient who is not taking anti-coagulant drugs has signs or symptoms of a bleeding disorder, which can range from nosebleeds, bleeding gums, bruising, heavy menstrual periods, blood in the stool and/or urine to arthritic-type symptoms (damage from bleeding into joints), loss of vision, and chronic anemia.

    Sometimes the PT may be ordered when a patient is to undergo an invasive medical procedure, such as surgery, to ensure normal clotting ability.

  • What does the test result mean?

    The test result for PT depends on the method used, with results measured in seconds and compared to the average value in healthy people. Most laboratories report PT results that have been adjusted to the International Normalized Ratio (INR) for patients on anti-coagulant drugs. These patients should have an INR of 2.0 to 3.0 for basic “blood-thinning” needs. For some patients who have a high risk of clot formation, the INR needs to be higher - about 2.5 to 3.5. Your doctor will use the INR to adjust your drug to get the PT into the range that is right for you.

    If you are not taking anti-coagulant drugs and your PT is prolonged, additional testing may be necessary to determine the cause. A prolonged, or increased, PT means that your blood is taking too long to form a clot. This may be caused by conditions such as liver disease, vitamin K deficiency or a coagulation factor deficiency. Result of the PT is often interpreted with that of the PTT in determining what condition may be present.

    Interpretation of PT and PTT in Patients with a Bleeding Syndrome

    PT resultptt resultPossible condition present
    Prolonged Normal Liver disease, decreased vitamin K, decreased or defective factor VII
    Normal Prolonged Decreased or defective factor VIII, IX, or XI, or lupus anticoagulant present
    Prolonged Prolonged Decreased or defective factor I, II, V or X, von Willebrand disease, liver disease, disseminated intravascular coagulation (DIC)
    Normal Normal Decreased platelet function, thrombocytopenia, factor XIII deficiency, mild deficiencies in other factors, mild form of von Willebrand’s disease
  • Is there anything else I should know?

    Some substances you consume, such as alcohol, can affect the PT/INR test. Some antibiotics can increase the PT/INR. Barbiturates, oral contraceptives and hormone-replacement therapy (HRT), and vitamin K - either in a multivitamin or liquid nutrition supplement - can decrease PT. Certain foods (such as beef and pork liver, green tea, broccoli, chickpeas, kale, turnip greens, and soybean products) contain large amounts of vitamin K and can alter PT results. Make sure that your doctor knows all the drugs you are taking and if you have eaten any of these foods recently so that your PT/INR results are interpreted and used correctly.

  • Can I do this test at home?
    Yes. The Food and Drug Administration has approved several home PT/INR testing systems. However, home testing is usually done in the context of a home-based coagulation management program that involves patient training and defined response and management protocols. For more on this, see the article on Home Testing.
  • Should I have it done at the same time of day?
    It is not generally necessary to have your PT/INR measured at a particular time of day. It is, however, important that you take your anti-coagulation medication at the same time each day to maintain a continuous level. If your doctor increases or decreases your dosage, she may want you to have your blood rechecked in a day or so to judge the effect of the dosage change on your PT/INR (it is not an immediate effect).
  • My PT results vary sometimes, yet my doctor doesn’t change my prescription. Why?
    Illness, change in diet, and some medications (as mentioned above) can alter PT results. Certain foods, such as beef and pork liver, green tea, broccoli, chickpeas, kale, turnip greens, and soybean products contain large amounts of vitamin K and can alter PT results. The blood collection technique and the difficulty in obtaining the blood sample can also affect test results. If your doctor has concerns about the stability of your PT/INR, he may test your blood more frequently.
View Sources
Sources Used in Current Review

Henry’s Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007, pp 733-737.

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University of Alabama at Birmingham Coagulation Service. Bleeding Guidelines. Available online at http://coag.path.uab.edu/bleeding.htm. Accessed August 2008.

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