Abnormal platelet function in myeloproliferative disorders.

Platelet aggregation and circulating platelet aggregates (CPAs) were evaluated in 18 patients with myeloproliferative disorders, both with and without thrombocytosis. No specific patterns of platelet aggregation were detected, but 11 of 18 patients demonstrated abnormal aggregation to epinephrine, nine of 18 had abnormal aggregation to adenosine diphosphate, and seven of 18 had abnormal aggregation to collagen. There was no definitive correlation of bleeding episodes with abnormal aggregation. However, significant bleeding was observed in a patient with a platelet count of 1,500,000/cu mm and abnormal aggregation. The aggregation defects persisted despite lowering of platelet count. Evidence of increased circulating platelet aggregates and normal platelet aggregation was seen in two patients, one of whom had transient cerebral ischemic attacks relieved by antiplatelet therapy, with return of the CPA index to normal. In two patients with digital ischemia, claudication, and angiographic evidence of peripheral vascular disease, no laboratory evidence of increased circulating platelet aggregates was observed, but one patient had regression of symptoms with antiplatelet therapy.