Polycythemia Vera

Polycythemia describes an increased haemoglobin or haematocrit; it is a finding, not automatically a diagnosis of polycythemia vera (PV). The elevation may be relative, from loss of plasma volume, or secondary to increased erythropoietin stimulation, such as with chronic hypoxia. PV is different: it is an acquired, BCR-ABL1-negative myeloproliferative neoplasm in which abnormal marrow signalling drives blood-cell production, most often through the JAK2V617F mutation. Constitutive activation of the JAK-STAT pathway keeps the marrow receiving growth signals even when normal feedback should reduce red-cell production. The resulting rise in red-cell mass increases blood viscosity and slows microvascular flow. The marrow disorder may also increase leukocytes and platelets, so the patient can face both thrombosis and haemorrhage rather than a simple “too many red cells” problem. The practical consequence is a circulation problem. Arterial or venous clotting can impair tissue perfusion, while abnormal platelet function or antithrombotic treatment can contribute to bleeding. Treatment therefore aims to lower haematocrit and reduce thrombotic risk without causing clinically significant cytopenias or haemorrhage.