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Polycythemia vera (PV) is a rare blood disorder that causes the body to make too many red blood cells. This can be caused by a JAK2 mutation. The high red blood cell count can make the blood thicker, increasing the risk of blood clots.
Established treatments for people with PV focus on maintaining hematocrit levels below 45 percent. Unfortunately, for some patients, current PV treatment options are not fully effective or are not tolerated and may cause serious side effects.
Hematocrit is the percentage of your blood that is made up of red blood cells. It is important for people with PV to keep their hematocrit levels below 45% to reduce the risk of blood clots. A blood clot can be dangerous, especially if it affects the lungs.
Reference: https://www.nccn.org/patients/guidelines/content/PDF/mpn-patient.pdf