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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 one of several blood coagulation tests. It measures how long it takes your blood to form a clot.

Normally, when one 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 monitor 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. The clotting factors involved are prekallikrein; high-molecular-weight kininogen; fibrinogen; and factors XII, XI, IX, VIII, II, V, and X. 

Why do I need this test?

You may need this test if your doctor suspects that you have a problem with one 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 monitor 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?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

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 25 to 41 seconds, but test results will vary depending on equipment and methods used. Therefore, standard normal results will differ in each lab. 

If your aPTT takes longer than usual, it may mean several things. Usually, 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 unusually short clotting time. If it does, it may be a sign of increased risk of thrombosis, bleeding, or multiple miscarriages.

If this test is done because you are taking heparin to help prevent blood clots, your doctor will usually want the aPTT to be about twice as long as what it would be normally. 

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

What might affect my test results?

Certain medicines, including heparin and warfarin (Coumadin), may affect the results of this test.

How do I get ready for this test?

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