DEFICIENCY OF FACTOR IX (HEMOPHELIA B)

• Biochemical aspects:
  • Factor IX is a single-chain glycoprotein with a molecular weight of approximately 50,000, containing about 20% carbohydrate.
  • Factor IX is activated by factor XIa. It is also activated by Russell's viper venom. Activated factor IX participates along with activated factor VIII, calcium and phospholipid in the activation of factor X. There is a reciprocity between factor IX and VII in that the activated form of each factor may activate the other.
• Pathological Basis:
  • Mode of inheritance: sex-linked recessive. The incidence of hemophelia is 1 in 10,000 people; 20% of these cases are hemophelia B.
  • Hemophelia B occurs in several forms, corresponding to the plasma level of factor IX activity: severe (less than 1%), moderate (1% - 5%), mild (5% - 25%).
  • The most serious complication in hemophelia B is the development of antibodies (inhibitors) to factor IX (10% of patients with hemophelia B).
• Treatment:
  • For patients with mild form of factor IX deficiency with non-life-threatening bleeding, the treatment of choice is FFP. For the severe form of hemophelia B or in life- threatening situation, prothrombin complex concentrates are the treatment of choice.
  • Target levels of factor IX for therapy:
    1. Severe hemorrhage: 20% - 50% for 3-5 days, then 10% - 20% for the next 10 days.
    2. Minor hemorrhage: 20% for 7 days.
    3. Surgery: 50% - 70% for 2 days, then 30% - 40% for 3 days, then 20% for 7 days.
  • Dosage: the (activity) units of factor IX required can be calculated from:
    • 80*BW*(desired factor level-actual factor level)/100
    • where BW is body weight in kg
    • Note that in-vivo recovery of factor IX is about 50%.