Partial Thromboplastin Time

Why It Is Done

Partial thromboplastin time (PTT) is done to:

• Find a cause of abnormal bleeding or bruising.
• Check for low levels of blood clotting factors. The lack of some clotting factors can cause bleeding disorders such as hemophilia.
• Check for conditions that cause clotting problems. Conditions such as antiphospholipid antibody syndrome or lupus anticoagulant syndrome develop when the immune system makes antibodies that attack blood clotting factors. This can cause the blood to clot easily in veins and arteries.
• Check if it is safe to do a procedure or surgery that might cause bleeding.
• Check how well the liver is working.

The activated partial thromboplastin time (APTT) test is used after you take blood-thinners to see if the right dose of medicine is being used. If the test is done for this purpose, an APTT may be done every few hours. When the correct dose of medicine is found, you will not need so many tests.

How To Prepare

Many medicines can change the results of this test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take.

How It Is Done

The health professional drawing blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure to the site and then a bandage.

How It Feels

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

• Taking some herbal products or natural remedies.
• Taking some medicines, such as antihistamines.

What To Think About

• The partial thromboplastin time (PTT) and activated partial thromboplastin time (APTT) may be normal in people who have inherited bleeding disorders but have only mild symptoms.
• The APTT is used to check treatment of people who are using heparin or other blood-thinning medicine to prevent blood clots.
• Sometimes people who use heparin have a higher APTT because of other substances in their blood and not because of blood-thinning medicines. A test called the heparin neutralization assay may be done to see if this is true.
• Another blood clotting test, called prothrombin time (PT), measures other clotting factors. Partial thromboplastin time and prothrombin time are often done at the same time to check for bleeding problems. To learn more, see the topic Prothrombin Time and INR.
• An PTT or APTT is done regularly in people who have bleeding or clotting problems. The tests are also done before procedures or surgeries where too much bleeding may be a concern.