Polycythemia verais caused by a clonal expansion of a multipotent myeloid stemcell that primarily produces extra erythrocytes.
Secondary polycythemia refers to increased red cell mass due to compromised abil-ity of blood to supply oxygen to tissues. Causes include chronic obstructive pulmo-nary disease and cyanotic congenital heart disease. Erythropoietin levels can be appropriately high. Secondary polycythemia may also be caused by inappropriately high erythropoietin levels, with renal cell carcinoma excreting erythropoietin being the typical cause.
Relative polycythemia refers to an increased red cell count secondary to decreasedplasma volume (typically due to dehydration). Red cell mass, erythropoietin, and blood oxygen content are normal.