Polycythemia is a condition that occurs when the bone marrow produces an abnormal and excessive amount of red blood cells and platelets in the blood. The abnormal increase of red blood cells in the blood can cause the blood to thicken and clot.

There are several forms of polycythemia—primary polycythemia, secondary polycythemia, and stress polycythemia. Each form has its own set of causes and risk factors.

Location of Active Bone Marrow in an Adult
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Nearly all cases of primary polycythemia are believed to be caused by a mutation in the janus kinase 2 gene (JAK2).