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Factor XII assay
Alternative namesThis is a blood test to measure the activity of factor XII -- one of the substances involved in coagulation (blood clotting).
How the test is performed
Blood is drawn from a vein, usually from 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.
How to prepare for the testThere is no special preparation needed for adults.
For infants and children:
The preparation you can provide for this procedure depends on your child's age, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics:
How the test will feelWhen 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
To determine the cause of abnormal results on a blood-clotting test (PTT).
Coagulation (blood clotting) results from a sequence of reactions involving proteins known collectively as the coagulation factors. The liver produces coagulation factors and secretes them into the blood.
Normal ValuesA normal value is 50-200% of the laboratory "control" or reference value.
What abnormal results meanDecreased factor XII activity may indicate:
What the risks are
Special considerationsFactor XII deficiency is not normally associated with clinical bleeding and Factor XII does not appear to be necessary for the formation of clots for normal hemostasis (stopping of bleeding).
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.
Update Date: 6/1/2003Marcia S. Brose, M.D., Ph.D., Division of Hematology/Oncology, Hospital of the University of Pennsylvania, Philadelphia, PA. Review provided by VeriMed Healthcare Network.
Last updated: Tue, 06 Jan 2009 00:20:03 GMT