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The prothrombin time Test Homestead Community Health Center

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.

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 prolongation of 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.


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.

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