Factor IX assay
The factor IX assay is a blood test that measures the activity of factor IX. This is one of the proteins in the body that helps the blood clot.
Christmas factor assay; Serum factor IX
How the Test is Performed
A blood sample is needed.
How to Prepare for the Test
You may need to stop taking some medicines before this test. Your health care provider will tell you which ones.
How the Test will Feel
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or slight bruising. This soon goes away.
Why the Test is Performed
This test is used to find the cause of too much bleeding (decreased blood clotting). Or, it may be ordered if a family member is known to have hemophilia B. The test may also be done to see how well treatment for hemophilia B is working.
A normal value is 50 to 200% of the laboratory control or reference value.
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or may test different samples. Talk to your doctor about the meaning of your specific test results.
The examples above show the common measurements for results for these tests.
What Abnormal Results Mean
Decreased factor IX activity may be related to:
- Congenital deficiency of factor IX (hemophilia B)
- Disorder in which the proteins that control blood clotting become over active (disseminated intravascular coagulation)
- Fat malabsorption (not absorbing enough fat from your diet)
- Liver disease (such as cirrhosis)
- Vitamin K deficiency
- Taking the blood thinning drug called warfarin (Coumadin)
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 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)
This test is most often done on people who have bleeding problems. The risk of excessive bleeding is slightly more than for people without bleeding problems.
Carcao M, Moorehead P, Lillicrap D. Hemophilia A and B. In: Hoffman R, Benz EJ Jr, Silberstein LE, Heslop HE, Weitz JI, Anastasi J, eds. Hematology: Basic Principles and Practice. 6th ed. Philadelphia, PA: Elsevier Saunders; 2013:chap 137.
Schmaier AH. Laboratory evaluation of hemostatic and thrombotic disorders. In: Hoffman R, Benz EJ Jr, Silberstein LE, Heslop HE, Weitz JI, Anastasi J, eds. Hematology: Basic Principles and Practice. 6th ed. Philadelphia, PA: Elsevier Saunders; 2013:chap 131.
Reviewed By: Yi-Bin Chen, MD, Leukemia/Bone Marrow Transplant Program, Massachusetts General Hospital, Boston, MA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.