Factor XII assay

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

How is the Test Performed?

A sample of blood will be taken from your vein.

Preparation for the Test

Adults do not have to take any special steps to prepare for this test.

How the Test will Feel

You may feel slight pain or a sting when the needle is inserted to draw blood. You may feel some throbbing afterward.

Why is the Test Performed?

Your doctor may want you to have this test if you had abnormal results on the partial thromboplastin time (PTT) blood-clotting test. You may also need the test if a family member was diagnosed with factor XII deficiency.

Normal Results for Factor XII assay

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:

Factor XII assay Risks

Veins and arteries vary in size so it may be harder to take a blood sample from one person than another.

Other slight risks from having blood drawn 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

Most of the time, people with factor XII deficiency do not have excess bleeding. Factor XII does not appear to be critical to stopping bleeding. However, one of the tests used to measure blood clotting in a test tube (PTT) does require factor XII. Longer than normal PTT results is what often triggers testing

References

Gailani D, Neff AT. Rare coagulation factor deficiencies. In: Hoffman R, Benz EJ Jr, Silberstein LE, Heslop HE, Weitz JI, eds. Hematology: Basic Principles and Practice. 6th ed. Philadelphia, PA: Saunders Elsevier; 2012:chap 139.

Ragni MV. Coagulation factor deficiencies. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Saunders Elsevier; 2011:chap 177.

Review Date: 3/3/2013
Reviewed By: Yi-Bin Chen, MD, Leukemia/Bone Marrow Transplant Program, Massachusetts General Hospital. Also reviewed by A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, Bethanne Black, Stephanie Slon, and Nissi Wang.
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