Polycythemia is sometimes called erythrocytosis, and there is significant overlap in the two findings, but the terms are not the same: polycythemia describes any increase in hematocrit and/or hemoglobin, while erythrocytosis describes an increase specifically in the number of red blood cells in the blood.

[6] Treatment of primary polycythemia (see polycythemia vera) could involve phlebotomy, antiplatelet therapy to reduce risk of blood clots, and additional cytoreductive therapy to reduce the number of red blood cells produced in the bone marrow.

[7] Polycythemia is defined as serum hematocrit (Hct) or hemoglobin (HgB) exceeding normal ranges expected for age and sex, typically Hct >49% in healthy adult men and >48% in women, or HgB >16.5 g/dL in men or >16.0 g/dL in women.

Relative polycythemia is often caused by loss of body fluids, such as through burns, dehydration, and stress.

In this syndrome, primarily occurring in obese men, hypertension causes a reduction in plasma volume, resulting in (amongst other changes) a relative increase in red blood cell count.

The production of red blood cells (or erythropoeisis) in the body is regulated by erythropoietin, which is a protein produced by the kidneys in response to poor oxygen delivery.

[15] As a result, more erythropoeitin is produced to encourage red blood cell production and increase oxygen-carrying capacity.

This results in secondary polycythemia, which can be an appropriate response to hypoxic conditions such as chronic smoking, obstructive sleep apnea, and high altitude.

A thorough cardiopulmonary exam including auscultation of the heart and lungs can help evaluate for cardiac shunting or chronic pulmonary disease.

polycythemia rubra vera (PRV)) occurs when excess red blood cells are produced as a result of an abnormality of the bone marrow.

In many families, PFCP is due to an autosomal dominant mutation in the EPOR erythropoietin receptor gene.

[30] In one study using the NHANES dataset, the prevalence of unexplained erythrocytosis is 35.1 per 100,000, and was higher among males and among individuals between ages 50–59 and 60–69.