Who is at risk of developing inhibitors?

Inhibitors occur more often in individuals with severe hemophilia than those with moderate or mild hemophilia and more often in hemophilia A than hemophilia B. Most patients who develop inhibitors do so within the period of the first 75 exposures to factor concentrates, with the greatest risk occurring between the first 10-20 treatments. This means that inhibitors occur mostly in children with severe hemophilia, though they can also occur in individuals with mild or moderate hemophilia following treatment.

  • About 25-30% of children with severe hemophilia A (factor VIII deficiency) develop inhibitors.
  • Only 1-6% of individuals with hemophilia B (factor IX deficiency) develop inhibitors.

Some people with hemophilia B who develop inhibitors may experience a severe allergic reaction called anaphylaxis if they continue to receive factor IX concentrates. Therefore, people with hemophilia B should be treated at a hemophilia treatment centre, particularly for the first 10-20 treatments with factor IX concentrates.

Ideally, children and adults who are newly diagnosed with hemophilia should be tested regularly for inhibitors between the 1st and 50th days of treatment. Even after the 50th day of treatment, they should be checked at least twice a year until they have received 150-200 doses and at least once a year after that. Testing for inhibitors should also be done before any major surgery.

Some other factors linked to higher risk of developing inhibitors include:

  • History of inhibitors in the family
  • Severe defects in the factor gene
  • African ancestry
  • Early intensive treatment with high doses of factor concentrates (particularly in the first 50 doses)

Some studies have shown that people who receive regular treatments with factor concentrates to prevent bleeds (prophylaxis or prophylactic treatment) have a lower chance of developing inhibitors. Little is known about whether the type of factor concentrates (recombinant or plasma-derived) that is used plays a role, but there are ongoing studies to shed light on this matter.

Studies on inhibitors are often small and more collaborative research is needed to improve the knowledge. 

Content developed by the WFH Inhibitors Working Group

Updated December 2014


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