Abstract
Venesection of 10% of whole blood volume or plateletpheresis was performed in nine patients with chronic myeloproliferative disorders and in five normal control subjects. Before venesection, the patients showed impaired platelet aggregation in 33% of tests, most often in response to stimulation with 9 μmol adrenaline. After venesection, the platelet and megathrombocyte counts increased rapidly and excessively in most patients and platelet aggregation improved markedly. In some cases, spontaneous in vitro aggregation was seen at high platelet concentrations. In two patients impaired platelet aggregation with adrenaline was not corrected. The splenic platelet pool is thought to be the probable source of the new platelets.