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Who is at risk of developing inhibitors?

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

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

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

Ideally, children and adults who are newly diagnosed with haemophilia 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 leasts 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.