Increased sensitivity of factor IX to phenprocoumon as a cause of bleeding in a patient with antiphospholipid antibody associated thrombosis

Harbrecht U, Oldenburg J, Klein P, Weber D, Rockstroh J, Hanfland P (University of Bonn, Bonn; University of Würzburg, Würzburg; and Evangelische Diakonissen Anstalt Bremen, Bremen, Germany). Increased sensitivity of factor IX to phenprocoumon as a cause of bleeding in a patient with antiphospholipid antibody associated thrombosis. J Intern Med 1998; 243 : 73–77. We report one patient who presented with a spontaneous bleeding complication under phenprocoumon therapy. Oral anticoagulation was initiated due to deep‐vein thrombosis which was attributed to an antiphospholipid antibody syndrome. Coagulation analysis revealed a strong and selective reduction of factor IX (F IX) activity to 1%, whereas the other vitamin K‐dependent factors (II, VII, X), the prothrombin time and International Normalized Ratio (INR) were within the therapeutic range. After withdrawal of phenprocoumon, all vitamin K‐dependent factors including F IX normalized. Because the patient suffered from a recurrence of thrombotic events, he was re‐exposed to phenprocoumon and the disproportionate decline of F IX was observed again. These findings indicate an increased sensitivitiy of F IX to vitamin K antagonists, representing an uncommon mechanism associated with bleeding complications under oral anticoagulant treatment.