The prothrombotic effect of an elevated HCT in ET and PV patients. Elevated HCT can increase the thrombotic risk by multiple mechanisms: (1) it determines an increase in blood viscosity; (2) at high shear rates, the raise of red cell mass displaces platelets toward the vessel wall, thus facilitating shear-induced platelet activation and enhancing platelet–platelet interactions; (3) under the low shear rates, as in the venous bed, hyperviscosity can increase the thrombotic risk by causing a major disturbance to the blood flow; and (4) biochemical changes in cell membrane and intracellular content of red blood cells.