Prothrombin Time, International Normalized Ratio Test -Why am I having this test?

Prothrombin time (pro-time, PT) test may be ordered if:

• You have certain medical conditions that cause abnormal bleeding or blood clotting. These can include:
  • Liver disease.
  • Systemic infection (sepsis).
  • Inherited (genetic) bleeding disorders.
• You are taking a medicine to prevent excessive blood clotting (anticoagulant), such as warfarin.
  • If you are taking warfarin, you will likely be asked to have this test done at regular intervals. The results of this test will help your health care provider determine what dose of warfarin you need based on how quickly or slowly your blood clots. It is very important to have this test done as often as your health care provider recommends.

What is being tested?

A prothrombin time (pro-time, PT) test measures how many seconds it takes your blood to clot. The international normalized ratio (INR) is a calculation of blood clotting time based on your PT result. Most labs report both PT and INR values when reporting blood clotting times.

What kind of sample is taken?

A blood sample is required for this test. It is usually collected by inserting a needle into a blood vessel.

Tell a health care provider about:

• Any blood disorders you have.
• All medicines you are taking, including vitamins, herbs, eye drops, creams, and over-the-counter medicines. Do not stop, add, or change any medicines without letting your health care provider know.
• The foods you regularly eat, especially foods that contain moderate or high amounts of vitamin K. It is important to eat a consistent amount of foods rich in vitamin K. Let your health care provider know if you have recently changed your diet.
• If you drink alcohol. This can affect your lab results.

How are the results reported?

Your test results will be reported as values. Your health care provider will compare your results to normal ranges that were established after testing a large group of people (reference ranges). Reference ranges may vary among different labs and hospitals. For this test, common reference ranges are:

• Without anticoagulant treatment (control value): 11.0–12.5 seconds; 85–100%.
• INR: 0.8–1.1.

If you are taking warfarin, talk with your health care provider about what your INR result should be. Generally, an INR of 2.0–3.0 is desired for blood clot prevention. This depends on your medical conditions.

What do the results mean?

• A higher than normal PT or INR means that your blood takes longer to form a clot. This can result from:
  • Certain medicines.
  • Liver disease.
  • Lack of certain proteins that form clots (coagulation factors).
  • Lack of some vitamins.
• A lower than normal PT or INR means that your blood can form a clot easily. This can result from:
  • Supplements that contain Vitamin K.
  • Medicines that contain estrogen, such as birth control pills or hormone replacement.
  • Cancer.
  • Some blood disorders (disseminated intravascular coagulation).

Talk with your health care provider about what your test results mean.

If you are taking warfarin or another anticoagulant, your result ranges may be different. Talk to your health care provider about what your results should be.

Questions to ask your health care provider

Ask your health care provider or the department that is doing the test:

• When will my results be ready?
• How will I get my results?
• What are my treatment options?
• What other tests do I need?
• What are my next steps?

Summary

• A prothrombin time (pro-time, PT) test measures how many seconds it takes your blood to clot.
• You may have this test if you have a medical condition that causes abnormal bleeding or blood clotting, or if you are taking a medicine to prevent abnormal blood clotting.
• A test result that is higher than normal indicates that your blood is taking too long to form a clot. This result may occur because you lack some vitamins, take certain medicines, or have certain medical conditions.
• Talk with your health care provider about what your results mean.