The blood platelet or thrombocyte concentration varies between individuals but remains rather constant in each individual. The reference range is 150-360 x 109/l. Thrombocytosis (platelet count > 360 x 109/l) can be classified into two categories: primary thrombocytosis, i.e. thrombocytosis related to myeloproliferative diseases reactive or secondary thrombocytosis. In primary thrombocytosis the patient may have symptoms and complications due to the high thrombocyte concentration. In reactive thrombocytosis, not even a high thrombocyte count causes symptoms but the symptoms and complications are related to the primary disease.