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
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:
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
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.