Factor IX Deficiency (Hemophilia B)

Deficiency of Factor IX (Hemophilia B) is the second most common type of hemophilia.  It is also known as Christmas disease after the first person diagnosed with the disorder in 1952.  It is largely an inherited disorder in which the FIX coagulation protein, necessary for normal blood clot formation, is missing or reduced.

The disorder is transmitted as an X-linked recessive trait, with the defective gene located on the X chromosome. Females have two copies of the X chromosome, so if the FIX gene on one chromosome is defective, the other can compensate. Males, however, have only one X chromosome, so if the FIX gene on that chromosome is defective, they will have the disease. Thus, more males than females have hemophilia. In about 30% of cases, there is no family history of the disorder and the condition is the result of a spontaneous gene mutation.

Females with one defective FIX gene are carriers of Hemophila B (they don't have symptoms). The male babies of carrier females have a 50% chance of having the disease, while their female babies have a 50% chance of being a carrier.

The severity of symptoms can vary, and the severe forms become apparent early in life. Bleeding is the main symptom of the disease and sometimes, although not always, occurs if an infant is circumcised. Additional bleeding problems usually show up when the infant becomes mobile. Mild cases may go unnoticed until later in life when they occur in response to surgery or trauma. Internal bleeding may occur anywhere and bleeding into joints is common.

In the laboratory, FIX deficiency gives rise to a prolonged Activated Partial Thromboplastin time (APTT) that is corrected on addition of normal plasma. The diagnosis of FIX deficiency is confirmed by means of a specific clotting assay for FIX. The normal reange for FIX clotting activity in the Florida Hospital Thrombosis Research Center is 70-130%.

In some patients with Hemophila B, autoantibodies to FIX may arise. These are detected in a manner similar to that used for FVIII inhibitors.