Schematics of spun hematocrit (Hct) on normal sample (left most tube), patient with polycythemia (normal mean cell volume [MCV] and elevated red blood cell [RBC] count, second tube from left), patient taking hydroxyurea (macrocytosis and reduced RBC count; third tube from left), and patient with thalassemia (microcytosis and elevated RBC count; right most tube), otherwise known as microcytic erythrocytosis.

Inverse correlation between mean cell volume (MCV) and red blood cell count (RBC) to maintain a constant hematocrit (Hct).

Today, the term erythrocytosis is generally reserved for cases of elevated red cell mass and is used synonymously with the term polycythemia. However, there are some exceptions (to beware of). For example, consider the following is from an abstract in a 2015 paper in the journal, Transfusion:

Transfusion 2015;55;2142–2148
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