Factor IX Activity

Factor IX is synthesised by the liver and has an intermediate lifespan (half-life of around 20 to 28 hours). In the coagulation cascade, FVIII combines with FIX to activate factor X. These three factors combine to form the tenase complex. The tenase complex is the main route for synthesising thrombin and then fibrin to form a solid clot. Factor FIX requires vitamin K to be active; it is said to be vitamin K-dependent. It can, therefore, be reduced in the event of vitamin K deficiency, as can factors II, VII, and X. The normal level of FIX is between 50 and 150%; haemophilia occurs when the level of FIX is less than 40%.

Haemophilia is a genetic disease, present from birth and transmitted within the family. Haemophilia is said to be X-linked recessive. The FIX gene is present on the X chromosome. Boys have only one X chromosome; if they are carriers of the genetic anomaly, they will be haemophiliacs. Girls have two X chromosomes, the second of which compensates for the deficiency, but only imperfectly. Among women with the genetic anomaly (or carriers), 2/3 will have normal factor levels, and 1/3 will have a deficiency, but generally less severe than in boys from the same family.

Haemophilia, therefore, mainly affects men, but some women can also be affected.

• Sometimes, there is no known family history of the disease; it is said to be sporadic.
• Some patients may develop an acquired factor IX deficiency due to conditions such as liver disease, vitamin K deficiency or the presence of inhibitors (autoantibodies) directed against factor IX. The test can detect these deficiencies and guide treatment.
• The incidence of the disease is approximately 1/30,000 male births, with a similar distribution throughout the world.

The severity of the disease is usually proportional to the severity of the deficiency. In severe forms of the disease and in the absence of treatment, bleeding may occur spontaneously, even without trauma. Bleeding occurs mainly in the joints (haemarthrosis) and muscles (haematomas).

Patients with haemophilia B often require replacement therapy, in which factor IX concentrates are administered to prevent or control bleeding episodes. Factor IX activity testing is used to monitor the effectiveness of this therapy, ensuring that appropriate levels of factor IX are maintained in the blood.