In PV patients, high levels of IL-12 are associated with hematocrit; high IL-1β and HGF levels are associated with leukocytosis; low IL-6 and FGF (fibroblast growth factor) levels are associated with hemoglobin concentration; and low GM-CSF levels are associated with thrombocytosis (36). Here, IL6 is linked to acquired polycythemia vera.