Relative Polycythaemia

An increase in the haemoglobin level and red blood cell count is found in the presence of marked dehydration, when the blood becomes more concentrated. This can occur after persistent vomiting and diarrhoea, traumatic shock, and extensive burns, when, in all these conditions, fluid is lost in large quantities from the body.

There is a decrease in plasma volume, and in the total circulating blood volume, and the haemoglobin and red cell levels will return to normal with adequate correction of the dehydration. It can also occur, particularly in young men, and be associated with high blood pressure. "Apparent", "spurious", and "stress" polycythaemia are other names for this condition. Heavy smoking, excessive alcohol consumption, and obesity, may also play a role in this condition.

The number of red blood cells normally in the blood varies, and is higher in men than women. New born babies have a higher red cell count than adults.

If there is a higher number of red blood cells in the circulation than normal then a person is said to have erythrocytosis or polycythaemia. The opposite situation can occur, where there is a lower level of red blood cells than normal, and this condition is referred to as "anaemia".

Raised red blood cell counts can be found incidentally in people with no symptoms, in the early stages of polycythaemia.