Partial Thromboplastin Time

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Test Overview

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.

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.

Risks

There is very little chance of a problem from having a blood sample taken from a vein.

  • You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
  • In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.

Results

Partial thromboplastin time (PTT) is a blood test that measures the time it takes your blood to clot.

Normal

The normal values listed here-called a reference range-are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

Partial thromboplastin timefootnote 1
Partial thromboplastin time (PTT):

60-70 seconds

Activated partial thromboplastin time (APTT):

30-40 seconds

The heparin dose is changed so that the PTT or APTT result is about 1.5 to 2.5 times the normal value.footnote 1

Abnormal values

  • A longer-than-normal PTT or APTT can mean a lack of or low level of one of the blood clotting factors or another substance needed to clot blood. This can be caused by bleeding disorders, such as hemophilia or von Willebrand's disease.
  • A longer-than-normal PTT or APTT can be caused by liver disease, kidney disease (such as nephrotic syndrome), or treatment with blood thinners.
  • A longer-than-normal PTT may be caused by conditions such as antiphospholipid antibody syndrome or lupus anticoagulant syndrome. These conditions happen when the immune system makes antibodies that attack blood clotting factors. This can cause the blood to clot easily in veins and arteries.
  • The PTT can get longer when you are using heparin, so your PTT value needs to be closely checked. If you have a longer PTT, you may have a higher risk of bleeding.

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.

References

Citations

  1. Pagana KD, Pagana TJ (2014). Mosby's Manual of Diagnostic and Laboratory Tests, 5th ed. St. Louis: Mosby.

Other Works Consulted

  • Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
  • Fischbach F, Dunning MB III (2015). A Manual of Laboratory and Diagnostic Tests, 9th ed. Philadelphia: Wolters Kluwer Health.
  • Pagana KD, Pagana TJ (2014). Mosby's Manual of Diagnostic and Laboratory Tests, 5th ed. St. Louis: Mosby.

Credits

ByHealthwise Staff

Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine

Specialist Medical ReviewerMartin J. Gabica, MD - Family Medicine

Current as ofApril 3, 2017

Current as of: April 3, 2017

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & Martin J. Gabica, MD - Family Medicine