Testing for inhibitors

An inhibitor is suspected whenever a bleeding episode is not controlled by a person's usual dose of replacement factor.1

How inhibitors are discovered

Inhibitors are usually found in one of two ways: during a routine inhibitor screening or when a replacement factor no longer stops or prevents bleeding.1 Inhibitor levels vary from person to person, and over time, inhibitor levels can even vary within the same person.2

Bethesda assay tests1

When your doctor wants to screen for inhibitors, you will be given a blood test called a Bethesda assay.

The Bethesda assay detects if an inhibitor is present in your blood and, if so, the amount of inhibitor present.

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Two categories of inhibitors

Inhibitors are divided into 2 categories based on the Bethesda assay and whether the inhibitor level increases when a person is treated with replacement clotting factor. The amount is called a titer, and it is measured in Bethesda units per milliliter (BU/mL).1


Test results of less than 5 BU/mL are called “low-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. Low-titer inhibitors may even be transient and disappear.1


Test results of 5.0 BU/mL and higher are called “high-titer” inhibitors.1

A person with high-titer inhibitors has more inhibitor present in the blood compared to a person with low-titer inhibitors.1

The hallmark of a high-titer inhibitor is “anamnesis,” which means that the inhibitor level increases over 4 to 7 days when a person’s blood is exposed to replacement factor.3

Keep exploring

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Treatment options for bleeding disorders with inhibitors

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  1. Srivastava A, Santagostino E, Dougall A, et al. WFH Guidelines for the Management of Hemophilia, 3rd edition. Haemophilia. 2020;26(suppl 6):1-158. https://doi.org/10.1111/hae.14046.
  2. Canadian Hemophilia Society. All about inhibitors. https://www.hemophilia.ca/files/All%20About%20Inhibitors.pdf. Accessed February 16, 2023.
  3. Kempton CL, White GC 2nd. How we treat a hemophilia A patient with a factor VIII inhibitor. Blood. 2009;113(1):11-17. doi:10.1182/blood-2008-06-160432