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Prothrombin Time and INR

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 therapy (HT), 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 broccoli, chickpeas, kale, turnip greens, and soybean products.
  • Drinking a lot of alcohol.
  • 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 warfarin 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. To learn more, 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.

Citations

  1. Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.

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.

WebMD Medical Reference from Healthwise

Last Updated: 2/, 014
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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