What is factor IX deficiency (FIX) (hemophilia B)?

Updated: Mar 09, 2021
  • Author: Robert A Schwartz, MD, MPH; Chief Editor: Srikanth Nagalla, MD, MS, FACP  more...
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Factor IX (FIX) deficiency or dysfunction, or hemophilia B, is an X-linked inherited bleeding disorder, usually manifested in males and transmitted by females who carry the causative mutation on the X chromosome. Hemophilia B results from a variety of defects in the FIX gene. FIX deficiency is 4-6 times less prevalent than factor VIII (FVIII) deficiency (hemophilia A).

Hemophilia B may be classified as severe, moderate, or mild, based on the plasma levels of factor IX in affected individuals (< 1%, 2-5%, 6-30%, respectively). [1] Multiple underlying mutations have been identified and linked with different levels of clinical severity. [2, 3]

Highly purified FIX concentrates are available for treatment of FIX deficiency. These include monoclonal antibody–purified plasma-derived FIX and recombinant FIX. See Treatment and Medication.

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