Laboratory Findings

Laboratory findings include platelet count in the range of 450,000-1,000,000/mm3, leukocytosis, basophilia, and the presence of megathrombocytes in the peripheral smear. Marrow cellularity is increased in 90% of the patients, with bizarre megakaryocytes with nuclear pleomorphisms and clustering of megakaryocytes. Enlargement of megakaryocytes with multilobulated nuclei, and their tendency to cluster in small groups along sinuses, is the hallmark of ET.21 The bone marrow may also appear normal.

Bleeding times are prolonged in 10-20% of the patients. Platelet aggregation studies are frequently abnormal, most often demonstrating impaired aggregation in response to epinephrine, ADP, and collagen, but not to arachidonic acid and ristocetin.22 Laboratory features of acquired von Willebrand syndrome (simulating type II vW factor deficiency) are associated with a platelet count >1000 X 109/L. An enhanced throm-botic risk in ET patients has been associated with a reduction in the concentration of protein S, antithrom-bin III, protein C, and resistance to activated protein C resulting from an associated genetic defect in factor V,23 along with patients who have both ET and acquired anticardiolipin antibodies.

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