Thrombocytosis

Key points

• Thrombocytosis is an abnormally high platelet count, usually > 400 * 109/l. Causes of thrombocytosis
• reactive: platelets are an acute phase reactant - platelet count can increase in response to stress such as a severe infection or surgery
• essential thrombocytosis (see below), or as part of another myeloproliferative disorder such as chronic myeloid leukaemia or polycythaemia rubra vera
• hyposplenism Essential thrombocytosis Essential thrombocytosis is one of the myeloproliferative disorders which overlaps with chronic myeloid leukaemia, polycythaemia rubra vera and myelofibrosis.
• Diagnostic threshold: the WHO criteria LOWERED the platelet threshold for essential thrombocythaemia from 600 to 450 × 10⁹/L (sustained), because 600 missed early disease · an older figure of > 600 × 10⁹/L is out of date.
• Driver mutations: JAK2 V617F (about half of cases), CALR, or MPL — demonstrating one of these, or excluding reactive causes, is part of the diagnostic criteria · management: aspirin, with hydroxycarbamide for higher-risk disease.