Factor IX assay
Christmas factor assay; Serum factor IX; Hemophilic factor B; Plasma thromboplastin component; PTC
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.
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.
Normal Results
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 provider 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:
- Hemophilia B (a bleeding disorder caused by a lack of blood clotting factor IX)
- Disorder in which the proteins that control blood clotting become overactive (disseminated intravascular coagulation)
- Fat malabsorption (not absorbing enough fat from your diet)
- Liver disease (such as cirrhosis)
- Vitamin K deficiency
- Taking blood thinners
Risks
There is little risk involved with having your blood taken. Veins and arteries vary in size from one person 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 lightheaded
- Multiple punctures to locate veins
- 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.
References
Carcao M, Gomez K, Davide M, Pierce GF. Hemophilia A and B. In: Hoffman R, Benz EJ, Silberstein LE, et al, eds. Hematology: Basic Principles and Practice. 8th ed. Philadelphia, PA: Elsevier; 2023:chap 134.
Pai M, Moffat KA. Laboratory evaluation of hemostatic and thrombotic disorders. In: Hoffman R, Benz EJ, Silberstein LE, et al, eds. Hematology: Basic Principles and Practice. 8th ed. Philadelphia, PA: Elsevier; 2023:chap 127.
Ragni MV. Hemorrhagic disorders: coagulation factor deficiencies. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 165.
Version Info
Last reviewed on: 2/2/2023
Reviewed by: Mark Levin, MD, Hematologist and Oncologist, Monsey, NY. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
