The Merck Manual (Definition)

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Polycythemia vera (primary polycythemia) is a disorder of the blood-producing cells of the bone marrow that results in overproduction of red blood cells.

- The cause is not known.
- People may feel tired and weak, light-headed, or short of breath.
- Blood tests are done for diagnosis.
- Phlebotomy is done to remove excess red blood cells.

In polycythemia vera, the excess of red blood cells increases the volume of blood and makes it thicker, so that it flows less easily through small blood vessels. Sometimes the spleen and liver also produce excess blood cells.