Polycythemia (or erythrocytosis) is the overproduction of red blood cells (erythrocytes). The mechanism of action resulting in polycythemia may be primary or secondary. Primary polycythemia indicates that the disorder arises at the level of the hematopoietic stem cells (see later), whereas secondary polycythemia represents the overproduction of RBCs caused by the increased stimulation of the bone marrow by EPO.

Polycythemia is typically identified initially in the laboratory by Hb and Hct values that are substantially above normal, requiring an explanation. Hb values of 18 to 20 g/dL lie in a range in which it is unclear whether the elevated Hb is caused by increased RBC production or a decreased plasma volume. In the latter case, the elevated Hb and Hct levels are caused by a contraction of the plasma volume and not by an increase in the RBC mass, so-called spurious polycythemia (pseudopolycythemia), or Gaisbock's syndrome, and may not warrant therapeutic intervention. Typically, Hb level above 20 g/dL is unequivocally associated with increased RBC mass.

In the evaluation of patients with polycythemia or suspected polycythemia, it is important to first establish that the RBC mass has increased. This is achieved through direct measurement of the red cell mass using isotope dilution techniques, and the test can be performed at most large hospitals. Once that issue has been resolved, and assuming that there is evidence of RBC overproduction, it is important to determine whether the polycythemia is primary or secondary by measuring the circulating EPO level. If elevated, causes of the increased EPO production to consider include heart or chronic pulmonary disease, in which there is desaturation of Hb as it leaves the lungs, or less common causes such as renal cysts, hepatic or cerebellar tumors, uterine leiomyoma, or impaired Hb function caused by heavy smoking, which results in elevated levels of carboxyhemoglobin, which is essentially inert as an oxygen transporter. Severe obstructive sleep apnea may cause enough desaturation to raise EPO levels, resulting in secondary polycythemia. Athletes using EPO, androgens, or blood doping may also present with secondary polycythemia.

Certain forms of polycythemia are inherited. These can be associated with mutations in the Hb molecule that increase Hb's affinity for oxygen, making it more difficult to offload oxygen within the tissues. These are referred to as "high-affinity mutants," and more than 20 have been identified. It is important to identify the various possibilities because different causative factors have different long-term consequences.

Primary polycythemia, or polycythemia vera (PV), is a disorder of the hematopoietic stem cell. It is acquired typically in midlife and thereafter and, when full-blown, is associated with an increase in the platelet count, white cell count, splenomegaly, and severe itching following hot showers or baths. Pruritus can be disabling and presumably results from degranulation of skin basophils. Unlike secondary forms of erythro-cytosis, EPO level is in the normal or low-normal range.

Treatment of PV strives to reduce the RBC mass to normal. Because PV is usually associated with an increase in total blood volume, the goal of phlebotomy therapy should be Hb and Hct levels in the normal range. Eventually, repeated phlebotomies induce iron deficiency, which is a reasonable end point. For patients with more aggressive disease (e.g., a growing spleen, increasing white cell and platelet counts), hydroxyurea therapy is effective. The goal is to maintain blood counts as close to normal as possible, or at least in an acceptable range.

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