Factor XII assay
This 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.
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
There 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:
- infant test or procedure preparation (birth to 1 year)
- toddler test or procedure preparation (1 to 3 years)
- preschooler test or procedure preparation (3 to 6 years)
- schoolage test or procedure preparation (6 to 12 years)
- adolescent test or procedure preparation (12 to 18 years)
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
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.
A normal value is 50-200% of the laboratory “control” or reference value.
What abnormal results mean
Decreased factor XII activity may indicate:
- congenital deficiency of factor XII
- liver disease
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
Factor 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.
by Martin A. Harms, M.D.
All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.