Factor XII Assay

Definition

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

Alternative Names

Hageman factor assay

How the test is performed

A blood sample is needed. For information on how this is done, see: Venipuncture.

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, or if one of your blood relatives was diagnosed with factor XII deficiency.

Normal Values

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.

The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.

What abnormal results mean

Decreased factor XII activity may indicate:

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

What the risks are

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)

Special 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 in your body. However, one of the tests used to measure blood clotting in a test tube does require factor XII.

References

Gailani D, Neff AT. Rare coagulation factor deficiencies. In: Hoffman R, Benz EJ Jr., Shattil SJ, et al, eds. Hoffman Hematology: Basic Principles and Practice. 5th ed. Philadelphia, Pa: Churchill Livingstone Elsevier; 2008:chap 127.

Kessler C. Hemorrhagic disorders: Coagulation factor deficiencies. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 180.

Review Date: 2/28/2011

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