Erythrocytosis (primary vs secondary)

Erythrocytosis simply means a raised red cell mass. Secondary erythrocytosis is the body's response to low oxygen or to excess erythropoietin and is not a cancer; primary erythrocytosis, of which polycythaemia vera is the main form, comes from a clonal marrow disorder. The two are separated by the serum erythropoietin level (low in PV, normal or high in secondary causes), the JAK2 mutation and the marrow picture. Getting this right matters because the treatments are entirely different.