polycytemia

Polycythemia, also called erythrocytosis, is a condition characterized by an increased red blood cell mass. It can be relative, due to a reduction in plasma volume (for example from dehydration or diuretic use), or absolute, due to an actual increase in red cell mass. Absolute polycythemia is subdivided into primary forms, such as polycythemia vera, and secondary forms driven by excess erythropoietin (EPO).

Primary absolute polycythemia refers to clonal disorders of the bone marrow, the most common being polycythemia vera (PV). PV is a myeloproliferative neoplasm often associated with mutations in the JAK2 gene. Secondary absolute polycythemia results from increased EPO production, typically in response to chronic hypoxia (lung disease, living at high altitude, smoking, sleep apnea, cyanotic heart disease) or tumors that secrete EPO. Rarely, exogenous factors such as androgens or therapeutic EPO can cause erythrocytosis.

Symptoms and signs largely reflect increased blood viscosity and the risk of thrombosis. Patients may experience headaches, dizziness, blurred vision, and a ruddy complexion. Aquagenic pruritus is common in PV. Splenomegaly can occur, and thrombotic events are a major concern.

Diagnosis relies on laboratory criteria. Elevated hemoglobin or hematocrit is detected, with confirmation of increased red cell mass. Testing for JAK2 mutations supports PV; erythropoietin levels are typically low in PV. Bone marrow biopsy may show hypercellularity with trilineage growth. The 2016 WHO criteria for PV require either all three major criteria or the first two major criteria plus a minor criterion (low EPO level or JAK2 mutation).

Management aims to reduce thrombotic risk and control symptoms. For PV, regular phlebotomy to keep hematocrit below about 45%, and low-dose aspirin are common. Cytoreductive therapy (eg, hydroxyurea) is used in higher-risk patients. Secondary polycythemia treatment focuses on addressing the underlying cause, such as relieving hypoxia or treating an EPO-producing tumor. Regular monitoring is essential to detect progression or complications.