Activated Partial Thromboplastin Clotting Time 

Does this test have other names?

Intrinsic pathway coagulation factor profile, aPTT, partial thromboplastin time, PTT, blood coagulation tests 

What is this test?

The aPTT is 1 of several blood coagulation tests. It measures how long it takes your blood to form a clot.

Normally, when 1 of your blood vessels is damaged, proteins in your blood called clotting factors come together in a certain order to form blood clots and quickly stop bleeding. The aPTT test can be used to look at how well those clotting factors are working. It's often used with other tests that keep track of clotting factors.

Blood clots form in a specific series of steps called a pathway. This test mainly looks at how both the intrinsic clotting pathway and the common final pathway are working.

Why do I need this test?

You may need this test if your healthcare provider believes that you have a problem with 1 or more clotting factors. If you have a bleeding disorder, such as von Willebrand disease or another disease that prevents your blood from clotting, this test can help find out where the problem is.

This test is also used to watch people who are getting heparin therapy. Heparin is a blood thinner used to prevent dangerous blood clots. 

What other tests might I have along with this test?

Depending on why you are having this test, you may have other blood tests that help measure how well your blood is clotting. Some of these tests might include:

  • Prothrombin time, or PT

  • Thrombin time, or TT

  • Activated whole blood clotting time, or ACT 

What do my test results mean?

Test results may vary depending on your age, gender, health history, and other things. Your test results may be different depending on the lab used. They may not mean you have a problem. Ask your healthcare provider what your test results mean for you.

Test results are measured in seconds of time. Your results will show how long your blood took to clot, and will often compare them with results from a normal sample tested at the same time. A normal range is around 21 to 35 seconds. But test results will vary depending on equipment and methods used. So standard normal results will differ in each lab. 

If your aPTT takes longer than normal, it may mean several things. Often other tests are done at the same time as the aPTT to better find out which factors are involved.

It's rare that your test will show an abnormally short clotting time. If it does, it may be a sign of increased risk for blood clots (thrombosis), bleeding, or multiple miscarriages.

If this test is done because you are taking heparin to help prevent blood clots, your healthcare provider will often want the aPTT to be about 2 to 2.5 times as long as what it would be normally. 

How is this test done?

The test is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand. 

Does this test pose any risks?

Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be sore. 

What might affect my test results?

Certain medicines may affect the results of this test. These include heparin and large doses of warfarin (Coumadin). If you are taking these medicines, follow your healthcare provider's directions. Your provider may want you to stop taking them before the test.

How do I get ready for this test?

You don't need to prepare for this test. Be sure your healthcare provider knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illegal drugs you may use.  

Online Medical Reviewer: Chad Haldeman-Englert MD
Online Medical Reviewer: Raymond Turley Jr PA-C
Online Medical Reviewer: Tara Novick BSN MSN
Date Last Reviewed: 9/1/2022
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