Platelet factor 4 and the response to plasma exchange in the treatment of thrombotic thrombocytopenic purpura.

We report two patients with thrombotic thrombocytopenic purpura who were subjected to plasma exchange. In one case, the plasma levels of platelet factor 4, measured shortly after plasma exchange, increased significantly during plasma exchange. This was followed, however, by a failure to respond to therapy. Repeated plasmapheresis over 3 weeks gave no therapeutic benefit and reversible deep coma occurred. This patient recovered completely after treatment with vincristine. In the second patient, a decline in platelet factor 4 was observed after plasma exchange. This was accompanied by improvement of the patient's condition and a slow rise in platelet count. Plasma exchange was again carried out in this patient because of a recurrence of thrombotic thrombocytopenic purpura 3 years later; again decreased platelet factor 4 plasma levels were observed after plasma exchange and again a therapeutic response followed. Platelet factor 4, therefore, seems to be an effective and early index for the therapeutic benefit of plasma exchange in thrombotic thrombocytopenic purpura.