Relative polycythemia is often caused by loss of body fluids, such as through burns, dehydration and stress.

Rarely, relative polycythemia can be caused by apparent polycythemia also known as Gaisböck syndrome.

Relative polycythemia is an apparent rise of the erythrocyte level in the blood; however, the underlying cause is reduced blood plasma.

It can be due to an increase in the number of red blood cells ("absolute polycythemia") or to a decrease in the volume of plasma ("relative polycythemia").