The Platelet Function Analyzer (PFA-100) may not be suitable for monitoring the therapeutic efficiency of von Willebrand concentrate in type III von Willebrand disease

We describe a type-III von Willebrand patient who was admitted to the hospi tal with severe deformity and functional deficit of the left knee joint due to recurrent hemarthrosis. Orthopedic intervention was necessary. To preve nt bleeding episodes, von Willebrand factor (VWF) replacement therapy was g iven during and after surgery. APTT, plasma FVIII activity (FVIIIc), VWF an tigen (VWF Ag), and vWF ristocetin cofactor (vWF Rco) were measured. Primar y hemostasis was monitored using the PFA-100. This "Platelet Function Analy zer" is designed to measure platelet. adhesion and aggregation capacities. Whole blood is aspirated through a capillary and is forced to flow through the central hole of a membrane coated with collagen and epinephrine (COL/EP I) or ADP (COL/ADP) as platelet activators. Irreversible platelet aggregati on results in the formation of a stable platelet plug, closing the central hole. The result is expressed as "closure time" (CT), i.e., time necessary to stop the blood flow, and is a measure of platelet hemostasis capacity. L aboratory investigations during substitution therapy revealed no shortening of closure times with both COL/EPI and COL/ADP cartridges despite normaliz ation of plasma vWF Ag, VWF Rco, and FVIIIc levels. These observations sugg est that intraplatelet vWF, which is totally absent in type-III von Willebr and disease, plays an important function in the adhesion of platelets to th e collagen-coated membrane of the PFA-100 system, simulating an injured ves sel wall. Consequently, we conclude that the PFA-100 may not be suitable fo r monitoring the therapeutic efficacy of von Willebrand concentrate in type -III von Willebrand patients during substitution therapy.