Inhibitor Testing—Bethesda Assay

Inhibitor Testing—Bethesda Assay

An inhibitor is suspected whenever a bleeding episode is not controlled by a patient's usual dose of replacement clotting factor.1 Inhibitors are also diagnosed in individuals without symptoms based on the results of routine blood tests.

Inhibitors are measured using a test called the Bethesda assay, which gives a numeric value.1 This number is quantified in Bethesda units per milliliter (BU/mL), and refers to the inhibitor titer or the antibody titer.

Are All Inhibitors the Same?

Inhibitors are divided into 2 categories based on the Bethesda assay and whether the inhibitor titer increases when a patient is treated with replacement clotting factor:

  • Low-responding inhibitors remain less than 5 BU/mL.2 These inhibitors may be transient and disappear. They may also be recurrent, coming and going throughout a person's lifetime.
  • High-responding inhibitors are equal to or greater than 5 BU/mL at any time. In the absence of replacement clotting factor exposure, high-responding inhibitors may decrease or become undetectable. However, when the patient is re-exposed to replacement clotting factor, his inhibitor titer will increase over 4 to 7 days. This increase is termed anamnesis and is a hallmark of the high-responding inhibitor.3

If You are Diagnosed With an Inhibitor...

If you are diagnosed with an inhibitor, your hemophilia treatment center (HTC) will work with you to create an individualized management plan that takes into consideration your current inhibitor titer. To learn more about living with inhibitors, visit



  1. Kasper CK. Diagnosis and Management of Inhibitors to Factors VIII and IX: An Introductory Discussion for Physicians. Montréal, Canada: World Federation of Hemophilia; 2004.
  2. White GC 2nd, Rosendaal F, Aledort LM, et al. Definitions in hemophilia. Recommendation of the scientific subcommittee on factor VIII and factor IX of the scientific and standardization committee of the International Society on Thrombosis and Haemostasis. Thromb Haemost. 2001;85(3):560.
  3. Kempton CL, White GC 2nd. How we treat a hemophilia A patient with a factor VIII inhibitor. Blood. 2009;113(1):11-17.

Inhibitor Testing—Bethesda Assay

Inhibitors are measured using a test called the Bethesda assay, which gives a numeric value in Bethesda units per milliliter (BU/mL).