Polycythemia is an increased number of red blood cells in the blood. In polycythemia, the levels of hemoglobin (Hgb), hematocrit (Hct), or the red blood cell (RBC) count may be elevated when measured in the complete blood count (CBC), as compared to normal. Hemoglobin levels greater than 16.5 g/dL (grams per deciliter) in women and greater than 18.5 g/dL in men suggest polycythemia. In terms of hematocrit, a value greater than 48 in women and 52 in men is indicative of polycythemia.
Production of red blood cells (erythropoiesis) occurs in the bone marrow and is regulated in a series of specific steps. One of the important enzymes regulating this process is called erythropoietin (Epo). The majority of Epo is produced and released by the kidneys, and a smaller portion is released by the liver.
Polycythemia can result from internal problems with the production of red blood cells. This is termed primary polycythemia. If polycythemia is caused due to another underlying medical problem, it is referred to as secondary polycythemia.
Most cases of polycythemia are secondary and are caused by another medical condition. Primary polycythemias are relatively rare; for example, polycythemia vera occurs in approximately 1 to 2 adults per 100,000 in the United States. One percent to five percent of newborns can have polycythemia (neonatal polycythemia).
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