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an increase in the number of platelets to more than 400,000 per mm3 in the peripheral blood. It may result from the increased production of platelets in the bone marrow, as in erythremia; from slow lysis of platelets, for example, after surgical removal of the spleen; or from the redistribution of platelets in the bloodstream during physical or mental stress. Thrombocytosis may lead to thrombosis, and occasionally to an increased tendency to bleed owing to disturbance of the microcirculation or to defects in the platelets themselves (thrombocytolytic purpura). Therapy involves treatment of the underlying disease, as well as the prevention of thrombosis.