The term thrombocytosis refers to a platelet count elevated above 450 × 109/litre, which can be (1) primary—including essential thrombocythaemia, chronic myeloid leukaemia, polycythaemia vera, and myelodysplastic syndromes; or (2) secondary—including iron deficiency, infection, blood loss, and malignancy. Essential thrombocythaemia: aetiology—the JAK2 V617F missense mutation typical of polycythaemia vera is found in about 50% of cases. In addition, 10% of patients have a mutation in the thrombopoietin receptor gene, MPL, and 30% have a mutation in calreticulin (CALR). Approximately 10% of patients have none of these mutations and are referred to as ‘triple negative’ essential thrombocythaemia. Diagnosis requires all of the following four major criteria: (1) platelet count greater than 450 × 109/litre; (2) bone marrow biopsy showing proliferation mainly of the megakaryocyte lineage with increased numbers of enlarged, mature megakaryocytes with hyperlobulated nuclei without a significant increase or left shift in neutrophil granulopoiesis or erythropoiesis and very rarely minor (grade 1) increase in reticulin fibres; (3) failure to meet the criteria for other myeloproliferative neoplasms; and (4) presence of JAK2, CALR, or MPL mutations. Alternatively, diagnosis can be met when the first three major criteria are present and the one minor criterion, namely the presence of another clonal marker or absence of evidence for reactive thrombocytosis. Treatment requires risk stratification based on the age of the patient and any prior history of thrombosis, with treatment being reserved for those at a high risk of developing complications and not introduced simply on the basis of platelet counts alone unless there is extreme thrombocytosis (>1500 × 109/litre). Therapies include low-dose aspirin and cytoreduction.
Modulation of JAK2 V617F allele burden dynamics by hydroxycarbamide in polycythaemia vera and essential thrombocythaemia patients
