Complications can also arise from inhibitors in the body. Factor VIII and factor IX inhibitors are circulating antibodies that actively destroy factor VIII or factor IX. These inhibitors are currently measured in Bethesda units (BU). One BU inhibitor is defined as the amount of inhibitor needed to neutralize half the factor VIII or factor IX in 2 hours in an equal mix of patient and normal plasma.

The most widely used incidence rate of inhibitors among patients with severe factor VIII deficiency is 10% to 30%.

What might indicate the presence of an inhibitor?

  • Use of higher or more frequent factor replacement doses
  • Failure to respond to infused factor replacement
  • Bleeding while on prophylaxis

Routine screening for the presence of inhibitors is usually performed

  • During the first months of factor use
  • During comprehensive hemophilia evaluations, once or twice a year
  • Prior to surgery

Inhibitors generally fall into two clinical categories, low responders and high responders.