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PT
Definition:
A test that measures the extrinsic clotting time of plasma.
Alternative Names:
Prothrombin time; Pro-time
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 for the test:
The health care provider may advise you to withhold drugs that can affect the test (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 experiences, 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 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:
The prothrombin time is tested to evaluate disorders of blood clotting, usually bleeding. It is a broad screening test for many types of bleeding disorders.
Normal Values:
The normal range is 11 to 13.5 seconds ("normal" varies somewhat in different labs).

For a person on full anticoagulant therapy the PT should be 2 to 3 times the laboratory "control" value.
What abnormal results mean:
Increased PT times may indicate:

This test may be performed under many conditions 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 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 a commonly-used anticoagulant drug. It acts by inhibiting the Vitamin K-requiring enzyme in the liver, which prevents formation of some of the clotting factors, thereby inhibiting coagulation.

When clotting factors contact damaged tissue, the coagulation sequence is initiated. Each clotting 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 involves activation of factor VII by thromboplastin (also called tissue factor), which is a protein released from the membranes of damaged tissues.

The PT is used to evaluate the adequacy of the extrinsic system and common pathway in coagulation. It measures the clotting ability of factors I (fibrinogen), II (prothrombin), V, VII, and X. When any of these factors is present in deficient quantities, the PT is prolonged.

Interfering factors
Anticoagulant therapy (heparin, warfarin)

Increased red blood cell mass. This results in excess anticoagulant in the serum (from the anticoagulant in the vial used to collect the sample), resulting in artificially prolonged PT.

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.
Review Date: 11/25/2001
Reviewed By: Rebecca Elstrom, M.D., Division of Hematology-Oncology, University of Pennsylvania Medical Center, Philadelphia, PA. Review provided by VeriMed Healthcare Network.

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