Polycythemia
(Elevated Red Blood Cell Count)
Medical Author: Siamak Nabili, MD, MPH
Medical Editor: Charles Davis, MD, PhD
What is polycythemia?
Polycythemia is a condition that results in an increased level of circulating
red blood cells in the bloodstream. People with polycythemia have an increase in
hematocrit, hemoglobin, or red blood cell count above the normal limits.
Polycythemia is normally reported in terms of increased
hematocrit or hemoglobin.
- Hematocrit (HCT): Polycythemia is considered when
the hematocrit is greater than 48% in women and 52% in men.
- Hemoglobin (HGB): Polycythemia is considered when a hemoglobin level of greater than 16.5g/dL in women or
hemoglobin level greater
than18.5 g/dL in men.
Polycythemia can be divided into two categories; primary and secondary.
- Primary polycythemia: In primary polycythemia the increase in red blood cells is due to inherent
problems in the process of red blood cell production.
- Secondary polycythemia: Secondary polycythemia generally occurs as a response to other factors or underlying conditions that
promote red blood cell production.
Red cell production (erythropoiesis) takes place in the bone marrow through a
complex sequence of tightly regulated steps. The main regulator of the red cell
production is the hormone erythropoietin (EPO). This hormone is largely
secreted by the kidneys, although, about 10% may be produced and secreted by the
liver.
Erythropoietin secretion is up-regulated in response to
low oxygen levels
(hypoxia) in the blood.
More oxygen can be carried to tissues when erythropoietin stimulates red blood cell
production in the bone marrow to compensate for the
hypoxia.
Neonatal (newborn) polycythemia can be seen in 1% to 5% of newborns. The most
common causes may be related to transfusion of blood, transfer of placental blood to the
infant after delivery, or chronic inadequate oxygenation of the
fetus (intrauterine hypoxia) due to placental insufficiency.
Next: What are normal ranges of hematocrit, red cell counts, and hemoglobin? »
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