Health Library

Factor IX (Antihemophilic Factor B)

Factor IX (Antihemophilic Factor B)

Does this test have other names?


What is this test?


The factor IX test is part of a larger screening to determine which type of hemophilia you have. If you lack the clotting substance called factor IX, you have a rare bleeding disorder found mostly in males. This disorder is called hemophilia B. It occurs only in one in 20,000 to 34,500 males worldwide.

Women carry the gene that causes this disease, but they may not have bleeding problems, except when they menstruate. A woman has a 50 percent chance of passing on the gene that causes the disorder to her children. 

Why do I need this test?

Hemophilia is an inherited disease, and testing is done to confirm that you have it. A health care provider may order this test if your family has a history of bleeding problems. If you're pregnant and have a strong family history, it's especially important to be tested before your baby is born.


If you're an adult with a history of unusual bleeding problems, your doctor may order this test to see if you have hemophilia. People with milder forms of hemophilia may not have problems until they are older.

If you are worried about having hemophilia but do not have bleeding problems, your doctor may order a genetic test. A genetic test can find the gene that causes hemophilia in most people.

Although hemophilia has no cure, it can be treated once a doctor knows which form of the disease is present.  

What other tests might I have along with this test?

The doctor may also order these other tests to learn more about your bleeding problems:

  • Activated partial thromboplastin time, or APTT, test. This test identifies clotting problems for factor IX, as well as other clotting substances.

  • Prothrombin time, or PT,  test. This test also identifies clotting problems for factor IX, as well as other clotting substances.


  • Fibrinogen test. This test determines the level of fibrinogen, a protein that helps blood to clot.

  • Clotting factor tests. These tests determine whether you have a bleeding disorder and how severe it is.  A person without a bleeding disorder has healthy clotting in 50 to 100 percent of his or her blood cells. If you have less than 50 percent, you may have hemophilia.


What do my test results mean?


This test will tell you if you have the rare form of a bleeding disorder known as hemophilia B. Talk with your doctor about whether you should be tested for other kinds of hemophilia. 

How is this test done?


Does this test pose any risks?

Giving a blood sample carries small risks such as bleeding, infection, bruising, and dizziness. When the needle pricks your arm, you may feel slight pain or stinging. Afterward, the site may be sore.

What might affect my test results?


How do I get ready for this test?

You don't need to prepare for this test. 

Health Library

Sign Up Today for Free e-Newsletters

Find a doctor or make an appointment:
General Information: (314) 747-3000
One Barnes-Jewish Hospital Plaza
St. Louis, MO 63110