Some Baiscs About Hemophilia Inhibitors

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Inhibitors Basics

Many people with hemophilia use treatment products named clotting factor concentrates (“factor”). These treatment products improve blood clotting and can also stop or prevent a bleeding episode. However, when a person develops an inhibitor, the body stops accepting the factor treatment product as a normal part of blood. The body begins to regard the factor as a foreign substance and tries to destroy it with an inhibitor. The inhibitor keeps the treatment from working. Thus, it becomes more difficult to stop a bleeding episode. For a person who develops an inhibitor, special treatment will be required until their body stops making inhibitors. Inhibitors most often appear during the first 50 times a person is treated with clotting factor concentrates, but they can appear at any time.  

Risk factors and Causes

All persons with hemophilia are at risk of developing an inhibitor. By far, Scientists do not know exactly the causes of inhibitors. But multiple research studies have shown that people with certain types of hemophilia gene mutations are more likely to develop an inhibitor. Besides this, some studies have also found other characteristics that possibly play a role in increasing the risk of inhibitor development among people with hemophilia. These include the following:
  • Medication history (the frequency of taking clotting factor concentrates)
  • Family history of inhibitors
  • Increased frequency and dose of treatment
  • Black race or Hispanic ethnicity


Inhibitors are diagnosed with a blood test. The blood test measures if an inhibitor is present and the amount of inhibitor present (called an inhibitor titer) in the blood. Inhibitor titers are measured in Nijmegen-Bethesda units (NBU) if the lab test used was the Nijmegen-Bethesda assay (NBA), or Bethesda units (BU) if the lab test used was the Bethesda assay. A person with a high inhibitor titer has more inhibitor present in the blood compared to a person with a low inhibitor titer. Test results of 5.0 NBU/BU or lower are called “low titer” inhibitors, whereas test results of greater than 5.0 NBU/BU are called “high titer” inhibitors. People diagnosed with low titer inhibitors are more likely to have shorter and more successful inhibitor treatment than those with high titer inhibitors. For these reasons, it is important that all people with hemophilia or people who use clotting factor concentrates get tested for inhibitors at least once a year.  


Treatment for people who have an inhibitor is complex, and it remains one of the biggest challenges in the care of people with bleeding disorders. Some inhibitors, called “transient” inhibitors, may disappear on their own without treatment. If possible, a person with an inhibitor should consider seeking care at an HTC. HTCs are specialized healthcare centers that bring together a team of doctors (hematologists or blood specialists), nurses, and other health professionals experienced in treating people with bleeding disorders. Some treatments for people with inhibitors include the following:
  • High-Dose Clotting Factor Concentrates: People with low titer inhibitors may be treated with higher amounts or increased frequency of factor to overcome the inhibitor and yet have enough left over to form a clot.
  • Bypassing Agents: Special blood products, called bypassing agents, are used to treat bleeding episodes for people with high titer inhibitors. Instead of replacing the missing factor, they go around (or bypass) the factors that are blocked by the inhibitor to help the body form a normal clot. Close monitoring of people taking bypassing agents is important to make sure that their blood is not clotting too much or clotting in the wrong places in the body.
  • Products that mimic Factor VIII: This type of product works by replacing the function of factor VIII (8) without being affected by inhibitors, and can be used to treat and prevent bleeding episodes in people with hemophilia A. This treatment product can be given by injection under the skin.
  • Immune Tolerance Induction (ITI) Therapy: The goal of ITI therapy is to stop the inhibitor from blocking factor in the blood and to teach the body to accept factor as a normal part of blood. With ITI therapy, people receive large amounts of factor every day for many weeks or months.
All inhibitor treatment options require specialized medical expertise. Treatment can be costly, particularly bypassing agents and ITI. It is important that people who are being treated for inhibitors have their blood tested often to measure their inhibitor titers to be sure that the treatment is working. HTCs can serve a vital role in supporting patients who undergo intensive treatment regimens, such as ITI, for inhibitors. keyword: hemophilia inhibitor treatment; hemophilia treatment center; risks hemophilia