Decreased half-life time of plasma von Willebrand factor collagen binding activity in essential thrombocythaemia: normalization after cytoreduction of the increased platelet count.

Patients with essential thrombocythaemia (ET) exhibit a decrease of large von Willebrand factor (VWF) multimers in plasma, which is inversely related to the platelet count. In the present study we investigated whether the decrease of large VWF multimers in plasma with increasing platelet counts is the consequence of increased turnover of large VWF multimers in vivo. To that end we measured the half-life times of endogenously released VWF:Ag and VWF:CBA (collagen binding activity) after intravenous administration of desmopressin (DDAVP) to nine ET patients and nine control subjects (N). In addition, the half-life times of VWF:Ag and VWF:CBA were also measured in four ET patients after cytoreduction of the increased platelet count to normal or nearly normal values. Estimated half-life times of VWF:Ag did not differ between ET patients and normals (11.0+/-4.0 h v 12.4+/-2.5 h, P>0.05). Estimated half-life times of VWF:CBA were significantly lower in ET patients as compared with normal individuals (6.1+/-2.0 h v 8.4+/-2.5 h, P<0.05). After cytoreduction of the increased platelet count to (nearly) normal values in all four ET patients the half-life time of VWF:CBA significantly (P=0.014) increased from 5.2+/-1.2 h to 8.7+/-2.0 h. Our data suggest that platelets may play a role in the homeostasis of circulating von Willebrand factor, which may compromise normal haemostasis at fairly increased platelet counts.