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Blood Test: Factor IX Activity

Reviewed by: Larissa Hirsch, MD
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What Is a Blood Test?

By taking and testing a small sample of a person’s blood, doctors can check for many kinds of diseases and conditions. Blood tests help doctors check how the body’s organs are working and see if medical treatments are helpful. 

To help your child get ready for a blood test, find out if they need to fast (not eat or drink) or should stop taking medicines before the test. Explain what to expect during the test. If your child is anxious about it, work together on ways to stay calm.

What Is a Factor IX Activity Test?

Proteins called clotting factors help blood clot properly and help prevent too much bleeding. A factor IX activity blood test lets doctors see how well a protein called factor IX is working.

The body's clotting factors are numbered using the Roman numerals I through XIII. They work together in a special order, almost like pieces of a puzzle. When the last piece is in place, the clot develops — but if even one piece is missing or defective, the puzzle can't come together.

Factor IX, with factor VIII, is involved in the creation of a "net" that closes a torn blood vessel. An abnormal gene can make some kids not have enough factor VIII, causing the bleeding disorder known as hemophilia A. Not having enough factor IX is known as hemophilia B.

Why Are Factor IX Activity Tests Done?

Doctors order the factor IX activity test to help diagnose or monitor the treatment of hemophilia B. The test also can help find the reason for an abnormal result on other clotting tests, such as prothrombin time (PT) or partial thromboplastin time (PTT).

What if I Have Questions?

If you have questions about the factor IX activity test or what the test results mean, talk to your doctor.

Reviewed by: Larissa Hirsch, MD
Date reviewed: September 2021