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Partial Thromboplastin Time

Partial thromboplastin time (PTT) is a blood test that measures the time it takes your blood to clot. A PTT test can be used to check for bleeding problems.

Blood clotting factors are needed for blood to clot (coagulation). The partial thromboplastin time is an important test because the time it takes your blood to clot may be affected by:

  • Blood-thinning medicine, such as heparin. Another test, the activated partial thromboplastin time (APTT) test, may be used to find out if the right dose of heparin is being used.
  • Low levels of blood clotting factors.
  • A change in the activity of any of the clotting factors.
  • The absence of any of the clotting factors.
  • Other substances, called inhibitors, that affect the clotting factors.
  • An increase in the use of the clotting factors.

Another blood clotting test, called prothrombin time (PT) or INR (international normalized ratio), measures other clotting factors. Partial thromboplastin time and prothrombin time are often done at the same time to check for bleeding problems caused by a problem with the clotting factors.

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.

WebMD Medical Reference from Healthwise

Last Updated: September 06, 2012
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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