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Red cells constitute the vast majority of the cells in circulation in the human body. The amount of red cells has an influence on the viscosity of the blood. Red cell production is under exquisitely sensitive control. Any disruption of this system can lead to an increase in red cell production – erythrocytosis – leading to increased viscosity, with clinical consequences.
An absolute erythrocytosis is present when there is an increase in the red cell mass over 125% of that predicted for the individual’s body mass.1 This can be measured. If the haematocrit (Hct) level is over 0.60 in a male or 0.56 in a female, the red cell mass has been shown to be increased, and it can be assumed in these cases that there is an absolute erythrocytosis. Haemoglobin (Hb) above 18.5g/dl or Hct above 0.52 in a male are judged to be elevated and warrant further investigation. The equivalent figures for females are 16.5g/dl and 0.48. These numbers do not always translate to an absolute erythrocytosis, as has been shown in a comparative study,2 and it may be necessary to formally measure the red cell mass to establish the presence of an absolute erythrocytosis. Red cells contain haemoglobin, which supplies oxygen to the tissues. The oxygen supply to the tissues is under fine control and the hormone produced in response to hypoxia is erythropoietin (EPO). EPO is mainly produced by the kidneys. Any fall in oxygen levels in the tissues in the kidneys will result in increased EPO production and, consequently, an erythrocytosis.
When an erythrocytosis is established, it is necessary to look for a cause. An absolute erythrocytosis can be classified depending on its aetiology. Erythrocytoses are classified as primary if there is an intrinsic defect in the erythroid progenitor cells in the bone marrow, or secondary if the increased red cell mass results from factors external to the erythroid progenitor cell (i.e. increased EPO production from any cause, driving red cell production).
Primary and secondary erythrocytoses can be subdivided into congenital and acquired groups (see Table 1). Primary erythrocytoses will have an EPO level below normal, as the intrinsic defect in the progenitor is responsible for the red cell production, and the physiological response to this is depression of EPO levels. In secondary erythrocytoses, EPO levels will be normal (inappropriate for a raised Hb) or elevated as the EPO is driving the erythrocytosis.