Prothrombin time test results can be presented in two ways:

  • Prothrombin time in seconds. Prothrombin time is usually measured in seconds — the time it takes for your blood to clot. This way of determining prothrombin time creates results that will vary depending on the laboratory and the method used to test the blood, but a sample range is approximately 10 to 14 seconds. A number higher than average means it takes blood longer than usual to clot. A lower number means blood clots more quickly than expected.
  • Prothrombin time as a ratio. For people taking the blood-thinning medication warfarin (Coumadin), results are given as a number that represents a ratio called the international normalized ratio (INR). The INR is a formula that adjusts for differences in the chemicals used in different laboratories so that test results can be comparable.

    An INR range of 2.0 to 3.0 is generally effective for people taking warfarin who need full anticoagulation, but may need to be slightly higher in certain situations. If your INR is higher than this range, that means your blood clots more slowly than desired. A lower INR means your blood clots more quickly than desired. The INR is used only for people on oral anticoagulant therapy. It's not useful in people whose PT is higher for other reasons.

What your results mean

Clotting too slowly

If your prothrombin time test reveals that your blood is clotting too slowly, this can be caused by:

  • Blood-thinning medications, such as warfarin (Coumadin), heparin
  • Liver problems
  • Inadequate levels of proteins (factors) that cause blood to clot
  • Vitamin K deficiency
  • Congenital factor deficiency
  • Presence of coagulation factor inhibitors

Clotting too fast

If your prothrombin time test reveals that your blood is clotting too fast, this can be caused by:

  • Supplements that contain vitamin K
  • High intake of foods that contain vitamin K, such as liver, broccoli, chickpeas, green tea, kale, turnip greens and products that contain soybeans
  • Estrogen-containing medications, such as birth control pills and hormone replacement therapy
Nov. 14, 2012