The PTT mixing study is consistent with an inhibitor, and the pattern of immediate correction followed by prolongation after incubation for 60 minutes is characteristic of a factor VIII inhibitor. PFA closure times rule out platelet dysfunction, either ibuprofen induced or von Wille brand disease. Further workup of the “inhibitor” requires measurements of factor levels and inhibitor titer. The results in this patient are as follows: Factor XI activity = 85% Factor IX activity = 91% Factor VIII activity = 2%; non-linear pattern with dilutions Factor VIII inhibitor titer = 23 Bethesda U/mL These results confirm a factor VI II deficiency-based coagulopathy consistent with an acquired “strong” autoantibody to factor VIII (a factor VI II of 2% with non-linear dilutions and a titer of 23 Bethesda U/mL). n order to undergo surgery to relieve the hematoma and save the forearm from ischemic injury, the coagulopathy must be reversed. Since the high titer inhibitor will overwhelm any factor VIII infusions, an alternative “bypass” strategy must be employed-infusion of 1 20 1-tg/kg of recombinant Vila every 3 hours prior to and following surgery. The frequency of infusions can then be gradually decreased once hemostasis is attained. Most acquired factor VI II inhibitors are not associated with an underlying disease. However, given the modest lymphocytosis in this patient, a search for a lymphoprol iferative disease is in order. In this case, immunopheno typing reveals a CDS+ B-cell clone consistent with the diagnosis of CLL (see Chapter 22). Moreover, subsequent treatment with rituximab results in eradication of the inhibitor and return of the PTT to normal.