How are inhibitors diagnosed?

Inhibitors are often suspected during a routine blood test called the activated partial thromboplastin time (APTT). The APTT test measures how long it takes for blood to clot. When inhibitors are present, the blood takes longer to clot and does not coagulate fully, even after mixture with normal plasma.

In order to confirm the diagnosis, a Bethesda assay, or a modification thereof, the Nijmegen method, is performed. This test can determine the strength (titer) of the inhibitor. However, the Bethesda/Nijmegen method is not available in many laboratories because it requires specialized expertise.

Inhibitor levels vary from one individual to another and can also vary within the same person over time. The amount of inhibitors in a person's blood is measured in Bethesda Units (BU) and referred to as “high titer” (more than 5 BU) or “low titer” (less than 5 BU).

Generally, high titer inhibitors act strongly to quickly neutralize infused factor concentrates, while low titer inhibitors are weaker and act more slowly. However, this is not always the case.

Inhibitors are also classified as “low responding” or “high responding” according to how strongly the person's immune system reacts to factor concentrates based on memory from a previous encounter - this is called the anamnestic response. A high responder is someone whose titer has exceeded 5 BU at least once and in whom repeated exposure to factor concentrate will quickly trigger the formation of new inhibitors. A low responder is someone whose titer has never exceeded 5 BU and who will have a weaker inhibitor response to factor concentrates. 

High Titer InhibitorsLow Titer Inhibitors
  • >5 BU
  • Inhibitors act strongly
  • They quickly neutralize factor
  • <5 BU
  • Inhibitors act weakly
  • They slowly neutralize factor 
High RespondingLow Responding
  • Inhibitor titer exceeded 5 BU at least once
  • Repeated exposure to factor will quickly trigger new inhibitors
  • Inhibitor never exceeded 5 BU
  • Exposure to factor will more slowly trigger new inhibitors


Content developed by the WFH Inhibitors Working Group


Updated May 2012


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