Thrombocytosis is the presence of high platelet counts in the blood, and can be either reactive or primary (also termed essential and caused by a myeloproliferative disease). Although often symptomless (particularly when it is a secondary reaction), it can predispose to thrombosis in some patients.

Definition

• 95% of the population will have a platelet count between 100,000 and 450,000 per mm³. Thrombocytosis is defined as >500,000. Extreme thrombocytosis is defined as >1,000,000. Generally, a normal platelet count ranges from 150,000 and 450,000 per mm³. These limits, however, are determined by the 2.5th lower and upper percentile, and a deviation does not necessary imply any form of disease. Nevertheless, counts over 750,000 (and especially over a million) are considered serious enough to warrant investigation and intervention.