Factor XII assay

The factor XII assay is a blood test to measure the activity of factor XII -- one of the substances involved in blood clotting.

How the Test is Performed

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The area is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm. 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, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

How to Prepare for the Test

There is no special preparation needed for adults.

How the Test Will Feel

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.

Why the Test is Performed

Your doctor may order this test if you had abnormal results on the partial thromboplastin time (PTT) blood-clotting test.

Normal Results

A normal value is 50 - 200% of the laboratory control or reference value.

Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

What Abnormal Results Mean

Decreased factor XII activity may indicate:

  • Inherited (congenital) deficiency of factor XII
  • Liver disease

Risks

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.

Other risks associated with having blood drawn are slight but may include:

  • Excessive bleeding
  • Fainting or feeling light-headed
  • Hematoma (blood accumulating under the skin)
  • Infection (a slight risk any time the skin is broken)

Considerations

People with factor XII deficiency usually do not have excess bleeding. Factor XII does not appear to be needed for clots to form during the normal process of stopping bleeding.

Alternative Names

Hageman factor assay