Partial Thromboplastin Time

Does this test have other names?

PTT, activated partial thromboplastin time, aPTT, APTT

What is this test?

This test measures how well your body clots blood. Partial thromboplastin time (PTT) may also be used to help your doctor look more closely at an episode of inappropriate bleeding or blood clotting. It can also be used to watch the effect of medication prescribed to intentionally thin the blood, such as heparin.

Some people's blood takes a long time to clot. This condition could be caused by a blood disorder, such as hemophilia, or a liver problem.

The PTT finds out whether you have enough blood-clotting substances known as factors VIII, IX, XI, and XII. If you don't have enough of some of these factors, it may take longer for your bleeding to stop. If you have a severe injury, you might die from excessive bleeding.

PTT is an older name for this test. Today it is widely known as an activated partial thromboplastin time test, or aPTT. 

Why do I need this test?

You may need a PTT test before surgery to find out whether you are at increased risk for uncontrolled bleeding. Your doctor may also order this test if you have heart problems, are at risk for a stroke, and take drugs to prevent clotting. If you're pregnant, your health care provider may order a PTT test before your baby is born.

If you have a bleeding complication for an unknown reason, your doctor will need to look for the cause. The PTT test can help determine whether you lack certain clotting substances in your blood. People who don't have enough of specific clotting factors may have a bleeding disorder. One example of a deficiency in clotting factor is called hemophilia.

If you're a woman who carries a gene that causes hemophilia, you may not have the symptoms of hemophilia, but you could pass it on to your unborn child. If you do carry the gene, your baby will need to be tested shortly after birth. Even if no one in the family has bleeding problems, it's possible your child may have hemophilia. Approximately one-third of babies who test positive for hemophilia have no relatives with the illness. 

What other tests might I have along with this test?

If your health care provider suspects a bleeding disorder, he or she may order a series of screening and clotting tests to find out which type you have and how severe it is. Some of these may include:

• Prothrombin time, or PT, test. This test finds clotting problems for factors I, II, V, VII, and X.

• Complete blood count, or CBC. This test measures the number, size, and quality of your blood cells.

• Fibrinogen test. This test looks for fibrinogen, a clotting substance also known as factor I.

• Clotting factor tests. These will show how severe your bleeding disorder is and may help find out the type of hemophilia.  A normal person will have healthy clotting in 50 to 100 percent of his or her blood cells. If you have healthy clotting in fewer than 50 percent, you may have hemophilia. 

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.

Your results will show how long it takes for your blood to clot. A normal time frame for blood to clot is between 25 and 41 seconds. Because the severity of the clotting disorder varies, talk with your doctor about results that fall outside this range.

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. If you have a bleeding disorder, it could take awhile for the bleeding to stop. 

What might affect my test results?

The drugs you take will affect your test. If you take blood thinners, such as the drug Coumadin, over-the-counter anti-inflammatory medications, or other medications that prevent clotting, tell your doctor. 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.

How do I get ready for this test?

You don't need to prepare for this test.