Partial thromboplastin time (PTT)

Alternative names 
APTT; PTT; Activated partial thromboplastin time

Definition
This is a test that measures clotting time in plasma (the liquid portion of blood). It focuses on a specific pathway in the blood clotting process.

How the test is performed

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

How to prepare for the test
The health care provider may advise you to stop taking drugs that can affect test results (see “Special Considerations”).

For infants and children:
The preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics:

How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed
This test is used to evaluate a wide variety of disorders of blood coagulation. It is useful in the evaluation of disorders of both excessive clotting and excessive bleeding.

Normal Values

     
  • APTT: 25 to 35 seconds

The normal value will vary between laboratories.

     
  • Patients receiving anticoagulant therapy: 1.5 to 2.5 times control values

What abnormal results mean

Prolonged PTT may indicate:

     
  • Cirrhosis  
  • disseminated intravascular coagulation (DIC)  
  • factor XII deficiency  
  • Hemophilia A (factor VIII deficiency)  
  • Hemophilia B (factor IX deficiency)  
  • hypofibrinogenemia  
  • malabsorption (inadequate absorption of nutrients from the intestinal tract)  
  • von Willebrand’s disease  
  • lupus anticoagulant

This test may be used in the evaluation of a wide variety of disorders.

What the risks are

     
  • excessive bleeding  
  • fainting or feeling light-headed  
  • hematoma (blood accumulating under the skin)  
  • infection (a slight risk any time the skin is broken)  
  • multiple punctures to locate veins

Note: this test is often performed on people who may have bleeding problems; if so, the risk of bleeding and hematoma are slightly greater than for people without bleeding problems.

Special considerations

Coagulation (blood clotting) results from a sequence of reactions involving several proteins known as coagulation factors. Some of these factors have other names. For example, Factor I is also called fibrinogen, Factor II is prothrombin, and Factor XII is Hageman factor. The liver produces these proteins and secretes them into the blood. In addition, vitamin K is important to blood clotting because your body converts it into prothrombin.

Some people take warfarin to keep their blood from clotting. Warfarin inhibits prothrombin, thus interrupting the clotting cascade. Because of the link between vitamin K and prothrombin, people who take warfarin need to have consistent levels of vitamin K in their diet, as instructed by their doctor.

Coagulation begins when some of the coagulation factors contact damaged tissue. Each factor reaction triggers the next reaction, in a cascade. The final product of the coagulation cascade is the Blood clot.

Factor X can be activated by two separate sequences of chemical reactions. The factors involved in the two sequences are referred to as the intrinsic system and the extrinsic system. The intrinsic system involves activation of Hageman factor by tissue not normally in contact with blood, followed by sequential activation of factors XI, IX, and X, in the presence of factor VIII.

Last Edited: Aug 17. 2005

Johns Hopkins patient information

Last revised: December 6, 2012
by Simon D. Mitin, M.D.

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