PTT

How the Test is Performed:
Adult or child:
Blood is drawn from a vein ( venipuncture ), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (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 tourniquet 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 .

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:
The health care provider may advise you to withhold drugs that can affect test results (see "special considerations").

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:



How it Feels:
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 .

Risks:

  • excessive bleeding
  • fainting or feeling lightheaded
  • 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.

Normal Values:

  • APTT: 30 to 40 seconds
  • PTT (therapeutic level): 60 to 70 seconds
  • Patients receiving anticoagulant therapy: 1.5 to 2.5 times control values



Abnormal Results:
Prolonged PTT may indicate:

Additional conditions under which the test may be performed:



Cost:
The estimated cost is $23.

Special Consideration:
Coagulation results from a sequence (cascade) of reactions involving coagulation factors, for example, factor I , factor II. Some of these factors have other names, for example, Factor I ( fibrinogen ), Factor II (prothrombin), and Factor XII ( Hageman factor ). These proteins are produced in the liver and secreted into the blood. Some of the Factors (that is, II, VII, IX, and X) require vitamin K for their synthesis. Warfarin (Coumadin) is an "anticoagulant" drug. It acts in the liver by inhibiting the action of the vitamin K-requiring enzyme , thereby inhibiting coagulation.

The coagulation sequence is triggered when some of these factors contact damaged tissues. Each coagulation factor reaction triggers the next reaction. The final product of the cascade is the fibrin clot ( blood clot ).

Factor X can be activated by 2 separate sequences of chemical reactions. The factors involved in the 2 sequences are referred to as the intrinsic system and the extrinsic system. The intrinsic system involves activation of Hageman factors by tissue not normally in contact with blood, followed by sequential activation of factors XI, IX, and X, in the presence of factor VIII. The extrinsic system simply involves activation of factor VII by thromboplastin (also called tissue factor), which is a protein released from the membranes of damaged tissues.

The PTT is used to assess the intrinsic system and the common pathway of clot formation. It evaluates factors I (fibrinogen), II (prothrombin), V, VIII, IX, X, XI, and XIII. When one of these factors is deficient, the PTT is prolonged.

Interfering factors:

  • anticoagulant therapy: heparin, warfarin
  • high levels of vitamin C
  • drugs that can prolong PTT include antihistamines, chlorpromazine, and salicylates.
  • an increased red cell mass, which results in excess anticoagulant remaining in the serum (from the anticoagulant tube used to collect the sample). This will result in prolongation of the PTT or APTT

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.