Erythrocytosis and thrombocytosis are among the most common reasons for referral to haematology outpatients. Most cases are identified on routine blood counts as part of the investigation of non-specific symptoms. Erythrocytosis is a term used to describe a group of conditions with an increase in circulating red cell concentration leading to an increase in haematocrit and often an increase in haemoglobin concentration (polycythaemia). True erythrocytosis is commonly seen in isolation but may also be associated with thrombocytosis and leukocytosis in the myeloproliferative disease polycythaemia vera (PV). Isolated erythrocytosis may also be a manifestation of myeloproliferative disease but is commonly seen as a secondary response to elevated erythropoietin levels, most frequently because of chronic hypoxia. Recently, an acquired point mutation in the gene Janus kinase 2 (JAK2) has been identified in approximately 95% of cases of PV, and this is a significant development in terms of diagnosis and as a future target for therapy. Persistent thrombocytosis is the hallmark of another myeloproliferative disease: essential thrombocythaemia (ET). The same acquired point mutation in JAK2 is identified in approximately 40–50% of cases of ET. Transient reactive thrombocytosis (days to months) is a common response to a wide range of conditions including acute and chronic blood loss, iron deficiency, malignancy, sepsis, chronic inflammation and temporal arteritis. The identification of the myeloproliferative forms of erythrocytosis and thrombocytosis is important as appropriate therapy will reduce the lifetime risk of arterial and venous thromboembolic disease.
