Resistance to activated F IX concentrate (FEIBA).

A 10-year-old haemophiliac with an inhibitor to Factor VIII was treated by repeated infusion of an activated Factor IX concentrate (FEIBA). During therapy the patient was monitored with extensive coagulation studies, and the initial effect of FEIBA on the whole blood coagulation time was seen to disappear in spite of increased doses of concentrate. A fall in Factor IX activity and a rise of antithrombin III and anti-Factor Xa was recorded, while the titer of anti-F VIII remained unchanged. The exact nature of this FEIBA-resistant state was not revealed, however, it was speculated that a rise of natural inhibitors of the coagulation system could explain the findings.