Polycythemia is defined by a hematocrit and/or a hemoglobin above the normal range. Relative polycythemia must be distinguished from absolute polycythemia. In relative polycythemia, the erythrocyte mass is not increased. The increased hematocrit and/or hemoglobin value occurs because the plasma volume is decreased (e.g. dehydration). In absolute polycythemia, the erythrocyte mass is increased.
The erythrocytes are tightly packed and overlap each other.
It is important to distinguish the primary form from the secondary form. With the primary form, polycythemia vera is normally present, which is a myeloproliferative disorder. The secondary form develops due to an increased production of erythropoietin caused by chronic hypoxemia (e.g. chronically obstructed pulmonary disease) or (rarely) with an erythropoietin-producing tumor (e.g. kidney tumor, kidney cyst).