At a Glance

Why Get Tested?

A prothrombin time (PT) is a test used to help detect and diagnose a bleeding disorder or excessive clotting disorder; the international normalized ratio (INR) is calculated from a PT result and is used to monitor how well the blood-thinning medication (anticoagulant) warfarin (Coumadin®) is working to prevent blood clots.

When To Get Tested?

When you are taking warfarin or when you have unexplained or prolonged bleeding or inappropriate blood clotting

Sample Required?

A blood sample drawn from a vein; sometimes blood from a fingerstick

Test Preparation Needed?

None needed, although if you are receiving anticoagulant therapy, the blood sample should be collected before taking your daily dose.

What is being tested?

The prothrombin time (PT) is a test that helps evaluate your ability to appropriately form blood clots. The international normalized ratio or INR is a calculation based on results of a PT that is used to monitor individuals who are being treated with the blood-thinning medication (anticoagulant) warfarin (Coumadin®).

A PT measures the number of seconds it takes for a clot to form in your sample of blood after substances (reagents) are added. The PT is often performed along with a partial thromboplastin time (PTT) and together they assess the amount and function of proteins called coagulation factors that are an important part of proper blood clot formation.

In the body, when there is an injury and bleeding occurs, the clotting process called hemostasis begins. This process involves in part 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. There must be a sufficient quantity of each coagulation factor, and each must function properly, in order for normal clotting to occur. Too little can lead to excessive bleeding or bleeding disorder; too much may lead to excessive clotting.

In a test tube during a laboratory test, there are two “pathways” that can initiate clotting, the so-called extrinsic and intrinsic pathways. Both of these then merge into a common pathway to complete the clotting process.

  • The PT test evaluates how well all of the coagulation factors in the extrinsic and common pathways of the coagulation cascade work together. Included are: factors I (Fibrinogen), II (Prothrombin), V, VII and X.
  • The PT/INR may be done at the same time as a PTT, which evaluates the clotting factors that are part of the intrinsic and common pathways: XII, XI, IX, VIII, X, V, II (prothrombin), and I (fibrinogen) as well as prekallikrein (PK) and high molecular weight kininogen (HK).

The PT and PTT evaluate the overall ability to produce a clot in a reasonable amount of time and, if any of these factors are deficient in quantity or not functioning properly, the test results will be prolonged.

The PT is usually measured in seconds and is compared to a normal range that reflects PT 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 ranges also will fluctuate. To standardize results across different laboratories in the U.S. and the world, a World Health Organization (WHO) committee developed and recommended the use of the Internationalized Normalized Ratio (INR), calculated based on the PT test result, for people who are receiving the anticoagulant warfarin (Coumadin®). Warfarin is prescribed for people with a variety of conditions, such as deep vein thrombosis (DVT) and some cardiovascular diseases (CVD) like atrial fibrillation to “thin” their blood and prevent inappropriate clotting.

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 report both PT and INR values whenever a PT test is performed. The INR should be only applicable, however, for those taking the blood-thinning medication warfarin.

Common Questions

How is the test used?

The prothrombin time (PT) is used, often along with a partial thromboplastin time (PTT), to help diagnose the cause of unexplained bleeding or inappropriate blood clots. The international normalized ratio (INR) is a calculation based on results of a PT and is used to monitor individuals who are being treated with the blood-thinning medication (anticoagulant) warfarin (Coumadin®).

The PT and INR are used to monitor the effectiveness of the anticoagulant warfarin. Warfarin is prescribed for people with a variety of conditions to “thin” their blood and prevent inappropriate clotting. A healthcare practitioner will typically prescribe warfarin and measure how well that dose “thins” the blood using the PT/INR. The dose may be adjusted up or down depending on the result and to ensure the dose is sufficient in preventing clots without causing excessive bleeding. This balance requires careful monitoring.

Warfarin may be prescribed for conditions such as:

  • Irregular heartbeat (atrial fibrillation)
  • The presence of artificial heart valves
  • Deep vein thrombosis (DVT), pulmonary embolism (PE)
  • Antiphospholipid syndrome
  • Occasionally, in heart attacks with certain risk factors

The PT test may be used along with a PTT as the starting points for investigating excessive bleeding or clotting disorders. By evaluating the results of the PT and PTT together, a health practitioner can gain clues as to what bleeding or clotting disorder may be present. These tests are not diagnostic by themselves but usually provide information on whether or what further testing may be needed.

Examples of other testing that may be done along with a PT and PTT or in follow up to abnormal results include:

  • Platelet count – to determine if platelets are decreased, which can cause excessive bleeding
  • Fibrinogen testing – may be done to rule out a low level or dysfunction of fibrinogen as a cause of a prolonged PT
  • Coagulation factor tests – these measure the activity (function) of coagulation factors. They can detect reduced levels of the protein or proteins that don’t work properly (have reduced function). Rarely, the antigen level (quantity) of a coagulation factor may also be measured.
  • von Willebrand factor – sometimes ordered to help determine if von Willebrand disease is the cause of a prolonged PTT
  • Lupus anticoagulant testing – may be ordered to further investigate the cause of prolonged PTT and/or PT, particularly for patients with clotting disorders

Based on carefully obtained patient histories, the PTT and PT tests are sometimes selectively performed as pre-surgical or before other invasive procedures to screen for potential bleeding tendencies.

When is it ordered?

A PT and INR are ordered on a regular basis when a person is taking the anticoagulant drug warfarin to make sure that the drug is producing the desired effect.

The PT may be ordered when a person who is not taking anticoagulant drugs has signs or symptoms of excessive bleeding or clotting, such as:

  • Unexplained bleeding or easy bruising
  • Nosebleeds
  • Bleeding gums
  • A blood clot in a vein or artery
  • An acute condition such as disseminated intravascular coagulation (DIC) that may cause both bleeding and clotting as coagulation factors are used up at a rapid rate
  • A chronic condition such as severe liver disease that may affect hemostasis

PT, along with PTT, may be ordered prior to surgery when the surgery carries an increased risk of blood loss and/or when the person has a clinical history of bleeding, such as frequent or excessive nosebleeds and easy bruising, which may indicate the presence of a bleeding disorder.

What does the test result mean?

For people taking warfarin, most laboratories report PT results that have been adjusted to the INR. These people should have an INR of 2.0 to 3.0 for basic “blood-thinning” needs. For some who have a high risk of a blood clot, the INR needs to be higher – about 2.5 to 3.5.

For individuals who are not taking warfarin, the reference range for a PT depends on the method used, with results measured in seconds and compared to the normal range established and maintained by the laboratory that performs the test. This normal range represents an average value of healthy people who live in that area and will vary somewhat from lab to lab. Someone who is not taking warfarin would compare their PT test result to the normal range provided with the test result by the laboratory performing the test.

A prolonged PT means that the 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 (e.g., factor VII deficiency). The PT result 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 or Clotting Syndrome

Can I do this test at home?

Yes, if you will be taking warfarin for an extended period of time. The Food and Drug Administration has approved several home PT and 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.

What food and medications can affect PT and INR results?

Some antibiotics can increase the PT and INR. Barbiturates, oral contraceptives and hormone-replacement therapy (HRT), and vitamin K (either in a multivitamin or liquid nutrition supplement) may decrease PT.

Drinking alcohol can also affect 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. It is important that a healthcare provider knows about all of the drugs, supplements, and foods that you have consumed recently so that the PT and INR results are interpreted and used correctly.

I am being treated with warfarin (Coumadin®). Should I avoid eating foods that are rich in vitamin K and will limiting these foods cause an increased risk of vitamin K deficiency?

Warfarin works by reducing the available vitamin K for the liver to make several of the blood clotting factors. Thus, warfarin and vitamin K are antagonists—they work against each other. Significant increases or decreases in the amount of vitamin K a person consumes can affect how well that person’s dose of warfarin works in preventing blood clots without causing excess bleeding. So rather than avoiding foods rich in vitamin K, it is more important for you to consume a consistent amount of those foods each day. You can get the vitamin K you need as long as you are consistent about the amount you consume.

Should I have a PT done at the same time of day?

It is not generally necessary to have your PT and INR measured at a particular time of day. It is, however, important that you take your warfarin medication at the same time each day to maintain a continuous level. If your health care provider increases or decreases your dose, your provider may want you to have your blood rechecked within a few days to judge the effect of the dosage change on your PT/INR (it is not an immediate effect).

My PT/INR 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/INR 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/INR results. The blood collection technique and the difficulty in obtaining the blood sample can also affect test results. If your health care provider has concerns about the stability of your PT/INR, your provider may test your blood more frequently before adjusting your dose.


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