What is a PTT (partial thromboplastin time) test?

A partial thromboplastin time (PTT) test uses a blood sample to measure how long it takes for your blood to make a clot. Normally, when you get a cut or injury that causes bleeding, many different types of proteins in your blood work together to make a clot to stop the bleeding. These proteins are called coagulation factors or clotting factors.

If any of your clotting factors are missing, at a low level, or not working properly, your blood may:

  • Clot too slowly after an injury or surgery. If this happens, you have a bleeding disorder. Bleeding disorders can cause serious blood loss. Hemophilia is one type of bleeding disorder.
  • Clot too much and/or too quickly, even without an injury. This condition may lead to clots that block your blood flow and cause serious conditions, such as heart attack, stroke, or clots in the lungs.

A PTT test helps check a specific group of clotting factors. It helps show how much of these clotting factors you have and how well they’re working. A PTT test is often done with other tests that check clotting factors and how well they all work together.

Other names: activated partial thromboplastin time, aPTT, intrinsic pathway coagulation factor profile

What is it used for?

A PTT test is used to check for problems with a specific group of blood clotting factors. The test is done to:

  • Find the cause of too much bruising or bleeding.
  • Find the cause of clotting problems. Causes can include certain autoimmune diseasessuch as lupus and antiphospholipid syndrome (APS).
  • Monitor people taking heparin, a type of medicine that’s used to prevent and treat blood clots. PTT testing can help make sure the dose is safe and effective.
  • Check the risk for possible bleeding problems before surgery or medical procedures. (A PTT test is not always used as a routine test before surgery. It may be used for certain people who may have a risk for bleeding problems).

Why do I need a PTT test?

You may need a PTT test if you:

  • Have problems with bleeding or bruising and the cause is not known
  • Have a blood clot in a vein or artery
  • Have liver disease (your liver makes most of your clotting factors).
  • Have had several miscarriages
  • Have been diagnosed with a bleeding or clotting disorder and don’t know which clotting factors are involved
  • Are taking heparin (to check how this medicine is affecting you, your health care provider may use a PTT test or another test instead).

What happens during a PTT test?

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Will I need to do anything to prepare for the test?

You don’t need any special preparations for a PTT test.

Are there any risks to the test?

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?

Your PTT test results will show how much time it took for your blood to clot. Results are usually given as a number of seconds. A PTT test is often ordered along with another blood test called a prothrombin time (PT) test. A PT test measures other clotting factors that a PTT test doesn’t check. Your provider will usually compare the results of both tests to understand how your blood is clotting. Ask your provider to explain what your test results mean for your health.

In general, if your blood took longer than normal to clot on a PTT test, it may be a sign of:

  • Liver disease.
  • A lack of vitamin K.
  • Certain genetic disorders that you inherit from your parents. These disorders affect certain clotting factors and increase your risk of bleeding. They include:
    • Von Willebrand disease.
    • Hemophilia.
  • Too much heparin.
  • Certain types of leukemia.
  • Autoimmune diseases, such as antiphospholipid antibody syndrome or lupus anticoagulant syndrome. These diseases cause your body to make proteins called antibodies. The antibodies related to these diseases cause too much clotting. But the results of a PTT test may show a slow clotting time. That’s because the chemicals in the PTT test react with the antibodies in your blood sample. This chemical reaction makes the blood sample clot more slowly than the blood in your body. If your provider thinks that an autoimmune disease is causing a clotting problem, you will usually have other tests to make a diagnosis.

If your blood clotted faster than normal on a PTT test, it may be a sign of:

  • The early stage of disseminated intravascular coagulation (DIC). This rare but serious condition may develop if you have an infection or damage to organs or tissues that affects blood clotting. In the early stage, you have too much blood clotting. Later on, DIC starts to use up clotting factors in your blood, which leads to bleeding problems.
  • Cancer of the ovaries, colon, or pancreas that is advanced, which means the cancer has spread to other parts of the body and is unlikely to be controlled with treatment.