| Summary: | Abstract Background Bleeding is the most common complication of oral anticoagulants, due to inadequate dosing. Case presentation This report describes the clinical course of a patient who developed severe bleeding under therapy with phenprocoumon, despite an INR in the lower therapeutic range. Strikingly, aPTT was prolonged, while factor IX activity was significantly reduced. Acquired hemophilia was excluded, due to missing detection of inhibitors. Finally, sequencing part of the factor IX gene including nucleotide position c.110 revealed a hemizygous factor IX mutation c.110C > T p (Ala37Val). Conclusions In rare cases, missense mutations in factor IX propeptide are associated with severe bleeding complications. The substitution of alanin at position 37 to either valin or threonin (Ala37Val or Ala37Thr) leads to hypersensitivity to vitamin k antagonists.
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