An activated prothrombin complex concentrate (Autoplex) was infused for the treatment of 102 bleeding episodes in 18 hemophilia A patients with antibody to factor VIII who were previously known as high responders. Among 95 bleeding episodes treated with a single infusion of the concentrate [43–107 factor VIII correcting units (FECU)/kg], only 1 anamnestic response was observed. On the contrary, a significant rise in antibody titer occurred following 2 of 4 double (at 8–12 hr interval) and all 3 multiple (over3 –10 days) infusions of Autoplex. The occurrence of immune responses may be explained by the presence of factor VIII coagulant antigen (VIII:CAg) in the 26 batches of Autoplex tested by two-site immunoradiometric assay (10-–36 U/vial). The anamnestic response was not correlated to the batch used nor to the amount (0.6–7.8 U/kg/day) of VIII:CAg infused per day. However, exposure to the concentrate over several days appeared to be the major predisposing factor for an immune response.

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