Polycythemia/Erythrocytosis

The terms of erythrocytosis or polycythemia refer to an absolute increase in red cell mass (RCM). Given the limited availability of RCM measurements and recent discoveries of causative mutations in several related disorders, the latest diagnostic criteria have emphasized elevated levels of hemoglobin or hematocrit as more practical indicators for clinical diagnosis of polycythemia/erythrocytosis. Specifically, hemoglobin levels greater than 16.5 g/dL in men and 16.0 g/dL in women, or hematocrit level exceeding 49% in men and 48% in women indicate a potential diagnosis. In this chapter, we will examine polyclonal polycythemias/erythrocytosis, while briefly addressing polycythemia vera, a clonal hematopoietic stem cell disorder, solely in the context of differential diagnosis.