Prothrombin Time (PT) Test

A prothrombin time (PT) test uses blood samples to measure how quickly your blood forms a clot. Healthcare providers often do this test to monitor your prothrombin levels if you’re taking the blood thinner warfarin. This test is also used to investigate potential blood disorders. A high PT level means your body takes more time than normal to form blood clots.


What is a prothrombin time (PT) test?

A prothrombin time (PT) test uses blood samples to measure how quickly your blood forms a clot. If you’re injured and bleeding, your body launches a step-by-step process that creates clots that stop the bleeding. That process involves proteins, called clotting factors or coagulation factors. Prothrombin is one of several clotting factors that combine forces to create blood clots. But the clotting process doesn’t work if there’s not enough of each clotting factor and not all factors function as they should. A high PT level means it takes more time than usual for your body to form clots.


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Why would I need a PT test?

Healthcare providers often do this test to monitor your prothrombin levels if you’re taking the blood thinner warfarin (Coumadin®). Warfarin helps prevent blood clots, which can cause serious conditions such as deep venous thrombosis or pulmonary embolism.

I don’t take warfarin. Why do I need this test?

There are several reasons why your healthcare provider may do a prothrombin time test:

  • You’re having surgery and your healthcare provider wants to confirm your blood clotting process functions as it should. A prothrombin time test is one way to do that.
  • You’re developing bruises, having heavy nosebleeds or cuts that don’t stop bleeding. A PT test can uncover the cause so your healthcare provider can treat the problem.
  • You have symptoms of bleeding disorders. A prothrombin time test is a first step toward identifying potential problems.

Did my healthcare provider order a PT test because I may have a serious condition?

If you’re taking a prothrombin time test, it’s because your healthcare provider is gathering information so they can assess your situation. A PT test is an early step in that process. It’s understandable if you have questions. Ask your healthcare provider to explain the test process and objective. They’ll be glad to explain what this test may show and how that information may be helpful.

What’s the difference between a prothrombin time test and a partial thromboplastin time (PPT) test?

Healthcare providers use PT tests to check prothrombin levels, one of the coagulation factors that help form blood clots. A partial thromboplastin time test checks your plasma for coagulation factor abnormalities.


Test Details

Do I need to do anything to prepare for a prothrombin time test?

Some medications, supplements, food and alcohol may affect test results. Tell your healthcare provider if you’re taking certain medications or supplements. Ask your healthcare provider what you should avoid before your test. Examples of medications that may affect PT test results include:

  • Some antibiotics.
  • Barbiturates.
  • Oral contraceptives.
  • Hormone replacement therapy.
  • Aspirin.
  • Heparin.
  • Antihistamines.

What vitamin supplements can affect test results?

If you’re taking warfarin, vitamin C and K supplements may affect your test results.

What foods can affect results?

Foods that contain large amounts of vitamin K can affect results, including:

  • Beef and pork liver.
  • Green tea.
  • Chickpeas.
  • Kale.
  • Turnip greens.
  • Soybean products.
  • Asparagus.
  • Broccoli.

What does a PT test involve?

A prothrombin time test is a simple procedure. To do this test, your healthcare provider will swab your skin with alcohol. Then, they’ll place an elastic band around your upper arm and ask you to make a fist. Making a fist helps your blood flow more easily. You may feel a sting or prick when your healthcare provider inserts the needle. After your healthcare provider has finished drawing blood, they’ll put a small bandage on spot where they inserted the needle.

Results and Follow-Up

How soon will I receive my test results?

Typically, people receive their test results within a few hours to one day. Your healthcare provider will explain what the results mean and whether you’ll need additional tests.

Why do I have two sets of results?

Most laboratories report your PT test results as the international normalized ratio, or INR. Your INR result is a calculated measurement that adjusts for differences in laboratory testing processes.

What is a normal PT/INR range?

Your test result will show different ranges for PT and INRs The normal ranges for clotting are:

  • 11 to 13.5 seconds
  • INR of 0.8 to 1.1
  • INR of 2.0 to 3.0 is a general therapeutic range for people taking warfarin.

What happens if my PT INR is high?

If your INR is above 1.1, your blood is clotting more slowly than normal. Depending on your situation, your healthcare provider may order additional tests so they can diagnose and treat the problem.

Why would someone taking warfarin have high PT INR results?

Several things can affect prothrombin time test results for people taking warfarin. You may have high PT/IR results if:

  • You haven’t been taking the proper dose of warfarin.
  • You’ve taken over-the-counter medicines, such cold medicines, or vitamin supplements, that can interact with warfarin.
  • You’ve consumed food and drinks that can interact with warfarin, such as kale, spinach, cranberry juice and alcohol.

A note from Cleveland Clinic

If you’re injured and bleeding, your body races to form blood clots to stop the bleeding so you can begin to heal. When that doesn’t happen, healthcare providers use a prothrombin time (PT) test to find out why. You may have regular PT tests because you’re taking the blood thinner warfarin (Coumadin). Some people have PT tests as a first step toward diagnosing conditions, including blood disorders. Your PT test results will show if your prothrombin levels are normal or too high. You may have questions about what’s next. Your healthcare provider will have answers.

Medically Reviewed

Last reviewed by a Cleveland Clinic medical professional on 03/14/2022.

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