Factor XII assay is a blood test to measure the activity of factor XII -- one of the substances involved in coagulation (blood clotting).

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. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell 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.

In infants or young children:

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. A bandage may be applied to the puncture site if there is any bleeding.

There is no special preparation needed for adults.

When the needle is inserted to draw blood, you may feel moderate pain, or only a prick or stinging sensation. Afterward, there may be some throbbing.

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.

Decreased factor XII activity may indicate:

• Congenital deficiency of factor XII
• Liver disease

• 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. In addition, 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.