Diagnosing Inhibitors

Your doctor might suspect an inhibitor if standard factor replacement therapy stops working.

Signs of an inhibitor include:

  • A higher dose of factor than usual is needed to stop the bleeding
  • Treatment takes longer than usual to work
  • More than the usual number of treatments are needed to stop the bleeding

Inhibitors can also be detected during routine testing for inhibitors, which is usually done during yearly comprehensive care visits at a Hemophilia Treatment Center (HTC) and before surgery.

Measuring Inhibitors

Inhibitors are confirmed through a blood test called a Bethesda inhibitor assay. This test measures the level of antibody (inhibitor) activity when a person's blood is exposed to a specific clotting factor.

Titer

The antibody level is reported as Bethesda titer, which is measured in Bethesda units (BUs). The higher the Bethesda titer, the more antibodies are present in the blood sample, and the stronger the attacks on the factor.

Inhibitor Types

Inhibitors are classified by the way they respond to factor treatment. They are either low-responding or high-responding. However, the type of inhibitor a person has can change over time. Inhibitors can even be transient.

  • Low-responding inhibitors (Bethesda titer of less than 5 BUs) have a slow and weak response to the factor. Low-responding inhibitors make replacement therapy difficult, but bleeding can usually be stopped by increasing the dose of Factor VIII or Factor IX.
  • High-responding inhibitors (Bethesda titer of more than 5 BUs) react to therapy quickly with high levels of antibodies. For people with high-responding inhibitors, it is likely that replacement with the missing factor will not work. Alternative treatments must be investigated, such as the use of bypassing products.

Inhibitor Types
Type of Inhibitor Type of Immune Response Reaction to Use of Factor VIII or IX
Low-responding Weak Inhibitor levels rise slowly and remain below 5 BU
High-responding Strong Inhibitor levels rise to more than 5 BU