What Do Your PT, PTT, and INR Results Mean?

And what changes them?

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Tests that assess blood clotting, known as coagulation studies, include prothrombin time (PT), partial thromboplastin time (PTT), and international normalized ratio (INR). You may need one or more of these tests to monitor an illness, follow the effects of blood thinner therapy, or as part of pre-surgical blood testing.

Types of Coagulation Tests
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Heparin and Lovenox (enoxaparin) are referred to as blood thinners and Coumadin (warfarin) is an anticoagulant—these are among the most common drugs that affect blood clotting. Your doctor may instruct you to stop taking your blood thinner or anticoagulant for a period of time prior to having an invasive procedure.

Normal Values for Coagulation Tests

There is a normal range of values in coagulation studies for people who are not taking blood thinners.

These ranges differ from the target values when a person is taking a blood thinner.

TEST NORMAL RANGES*
PT 10-12 seconds
PTT  30-45 seconds
INR  1:2 ratio
*For people not on blood thinners

Blood thinners and anticoagulants will make blood take longer to clot. If you are taking one of these medications, your results will likely be higher than the standard normal values, reflecting the longer time that it takes for your blood to form a blood clot.

Prothrombin Time (PT)

The prothrombin time (PT) test measures how quickly your blood clots. Taking Coumadin is the most common cause of prolonged PT.

Other possible causes include:

  • Vitamin K deficiency
  • Disseminated intravascular coagulation (DIC), a serious clotting problem that requires immediate intervention
  • Liver disease
  • Hormone drugs, including hormone replacements and oral contraceptives

Additionally, the PT result can be altered by a diet high in beef or pork liver, green tea, dark green vegetables, and soybeans.

Partial Thromboplastin Time (PTT)

The partial thromboplastin time (PTT) test is performed primarily to determine if heparin therapy is effective and it can also detect a clotting/bleeding disorder. However, it can't be used to monitor the effect of drugs such as Lovenox, which are used to prevent and treat blood clots in the legs and complications of heart disease.

Several conditions and medications can impact your PTT test results, including:

Your doctor might ask you to stop taking certain drugs before your coagulation study. Do not stop taking any of your medications without your doctor's advice.

International Normalized Ratio (INR)

The international normalized ratio (INR) is used to make sure the results from a PT test are the same from one lab to another. In the 1980s, the World Health Organization determined that people undergoing surgery may be at risk of excessive bleeding because different labs had different standardized values than other labs.

The INR result should be the same, regardless of the location where the tests are performed.

Associated Risks and Complications

Blood draws are considered routine, low-risk procedures. While rare, it is possible to have complications from a simple blood draw.

These include:

  • Pain at the needle-insertion site
  • Bruising at the needle-insertion site
  • Lightheadedness or feeling faint
  • Anxiety over needles
  • Excessive bleeding, especially if you're taking blood thinners
  • Infection

If you feel lightheaded during or after a blood draw or the site continues to bleed, let someone at the facility know so they can assist or monitor you as needed. Keep your puncture site covered and clean for a few days and watch for signs of infection (pain, redness, heat, swelling, fever, chills) over the next couple of days. Get medical help if signs of an infection develop.

Frequently Asked Questions

What if my PT test shows an abnormal level?

After an abnormal PT test result, your doctor may want to run additional tests. If you are not currently taking any medication that affects blood clotting factors, you may need to start taking one to normalize your levels.

If you are taking a medication that affects clotting, your dosage may require an adjustment if your PT is not in the target range.

Can your diet impact your INR levels?

Yes, if you are taking warfarin, certain foods and drinks, that contain vitamin K, like dark leafy greens or green tea, may reduce its effectiveness. Alcohol or cranberry and grapefruit products can increase the risk of bleeding when taking warfarin.

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