Pseudo-von Willebrand's disease. An intrinsic platelet defect with aggregation by unmodified human factor VIII/von Willebrand factor and enhanced adsorption of its high-molecular-weight multimers.

Four members (from four generations) of a family with a mild bleeding disorder and intermittent thrombocytopenia had decreased plasma levels of properties related to factor VIII/von Willebrand factor (FVIII/VWF), an absence of high-molecular-weight forms of FVIII/VWF in the plasma (but normal multimeric structure in the platelets), and increased ristocetin-induced platelet aggregation, as in Type IIB von Willebrand's disease. However, unlike the abnormality in FVIII/VWF in Type IIB disease, the basic defect in this family was in their platelets, which absorbed FVIII/VWF high-molecular-weight multimers at lower concentrations of ristocetin than did normal platelets. In addition, either in platelet-rich plasma or suspended in buffer, their platelets were aggregated by unmodified normal human FVIII/VWF without ristocetin. Since the abnormalities of plasma FVIII/VWF in this family may be secondary to the platelet abnormalities, the term "pseudo-von Willbrand's disease" may be suitably descriptive of their disorder.