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Traditionally, polycythaemia describes a group of varied disorders with an increase in the number of circulating red cells above established normal limits (erythrocytosis is the appropriate term since only the red cell lineage is involved in the disease) (1).
Age, sex, nutritional status and residential altitude all contribute to natural variations. As a guideline, erythrocytosis should be suspected when there is persistently raised venous haematocrit (Hct) (>0.52 males, >0.48 females for > 2 months) or haemoglobin (in men greater than 18.0 g per L or in women with values greater than 17.0 g per L) (1).
The increased blood viscosity associated with polycythaemia increases the risk of thrombotic complications.
Erythrocytosis can be:
True increased red cell mass (rather than apparent erythrocytosis) can be assumed when the haematocrit is over 60% in males and 56% in females (2)
Note that normal ranges vary with different commercial kits.
A schemata for considering polycythaemia in adults is presented (3):