Blood tests that are needed when taking warfarin

What blood test do I need?

To find out how fast your blood clots, a small amount of your blood will be tested. This is taken from your finger or arm. This blood test is called an International Normalized Ratio (INR). Your health-care provider may also report your prothrombin time, which is also called protime (PT).

Why do I need blood tests?

Keeping your blood from clotting is a delicate process that is watched closely by your health-care provider. An INR range will be set for you.

  • If your INR stays within your range, your chance of bleeding or getting a clot is small.
  • If your INR is above your range, you may be at a higher risk for bleeding.
  • If your INR is below your range, you may be at a higher risk for making a blood clot.

This is why your blood needs to be tested often. Blood tests are done so your health-care provider will know if you are taking the right dose of warfarin. This keeps you safe from bleeding and making clots.

How often do I need an INR?

When you first start warfarin, you may need to have blood tests every few days or weekly. Once your dose is stable, blood tests are usually done every two to four weeks, sometimes longer. If your dose changes you may need to have your INR tested more often. It is very important to have your blood drawn when you are told to. Call your health-care provider if you can't make a visit so you can reschedule.

What can change my INR?

There are some things that may change your INR. It is important to tell your health-care provider about any of the changes below. This is so you can have an INR tested or your dose of warfarin changed if needed.

Some things that affect your warfarin are:

  • Illness, such as diarrhea or fever
  • Eating more or less foods that have vitamin K
  • Changes in your medicines (prescription, over- the-counter, herbs, or natural supplements)
  • Alcohol intake
  • Missing dose
  • Changes in your weight
Last reviewed: 
August 2017

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