Abnormality of glycoprotein Ib in two cases of "pseudo"-von Willebrand's disease.

Two unrelated patients with "pseudo" ("platelet-type")-von Willebrand's disease (vWD) are described demonstrating thrombocytopenia with a prolonged bleeding time, ristocetin-induced platelet aggregation at low stimulus concentrations, decreased levels of ristocetin-cofactor activity (vWF:RCo), slight cryoprecipitate-induced platelet aggregation in the absence of ristocetin, and a lack of the high molecular weight factor VIII-von Willebrand factor (FVIII/vWF) multimers in plasma. Isolated washed patient platelets bound more FVIII/vWF at high (1 and 0.75 mg/ml) and low (0.5 and 0.25 mg/ml) ristocetin concentrations than control platelets. Fresh or paraformaldehyde-fixed washed platelets from these patients also bound more specific monoclonal antibody to glycoprotein Ib (25,000 binding sites per cell) than normal platelets (15,000 +/- 3,300 binding sites per cell). Results obtained in control patients with thrombocytopenia and increased platelet size (May-Hegglin anomaly and vWD type IIB) excluded a nonspecific increase of glycoprotein Ib in the platelets of the patients with pseudo-vWD. These data indicate that in pseudo-vWD, the primary abnormality lies in the platelet and is related to a quantitative and/or qualitative anomaly of platelet membrane glycoprotein Ib.