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- National Center for Biotechnology Information - Polycythemia (Nov. 02, 2026)
polycythemia
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What is polycythemia?
Polycythemia is an abnormal increase of red blood cells and hemoglobin in the blood, leading to thickened blood, reduced flow, and a higher risk of clot formation. This condition is often the body's response to an increased need for oxygen, such as at high altitudes or due to impaired pulmonary ventilation.
Polycythemia can be relative, transient, or absolute. When the cause is known, absolute polycythemia is called erythrocytosis. A related condition, polycythemia vera, involves excessive red blood cells without a known cause, often with increases in white blood cells and platelets, and enlargement of the spleen. Treatment aims to reduce red blood cell volume through blood removal or, in some cases, medication or radiation.
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How does polycythemia affect the blood?
Polycythemia is a condition marked by an abnormal increase of red blood cells and hemoglobin in the blood. This thickening of the blood can slow its flow and elevate the risk of clot formation within the circulatory system. Polycythemia can be relative, transient, or absolute. Increased blood viscosity from polycythemia can cause skin disorders. In polycythemia vera, the number of red cells, white blood cells, and platelets are increased, and the spleen is typically enlarged.
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What are the symptoms of polycythemia?
Polycythemia, an abnormal rise in red blood cells, thickens blood, slows its flow, and elevates the risk of clot formation. This condition may result from the body's response to increased oxygen demand or may be caused by polycythemia vera.
Symptoms of polycythemia vera include a ruddy complexion, red discoloration of the face and sometimes the extremities, headache, dizziness, difficulty in breathing, a feeling of fullness, skin changes (e.g., tendency to bruise), and an enlarged spleen. Hemorrhages may occur from ulcers or minor wounds because certain blood-clotting factors are not produced in adequate amounts.
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What causes polycythemia to develop?
Polycythemia, an abnormal increase of red blood cells in circulation, arises from the body's response to increased oxygen demand or, in the case of polycythemia vera, from an unknown cause.
Known causes include:
* Decreased atmospheric pressure (high altitudes)
* Impaired pulmonary ventilation from chronic pulmonary disease or extreme obesity
* Congenital heart disorders that disrupt normal blood flow
* Defects in circulating hemoglobin, whether congenital or acquired through substances like coal tar derivatives
* Certain tumors or adrenocortical secretionsIn polycythemia vera, the stem cells in bone marrow produce excessive progeny, increasing red blood cells and often white blood cells and platelets, with the spleen usually enlarged.
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How do doctors diagnose polycythemia?
Doctors diagnose polycythemia (also called erythrocytosis) through blood tests that reveal an increase above normal in the number of red cells in the circulating blood, usually accompanied by an increase in the quantity of hemoglobin and in the volume of packed red cells. A complete blood count (CBC) measures hematologic parameters, calculating red and white blood cell counts in a cubic millimetre of blood.
An increase in red blood cells (polycythemia) is normal for persons living at high altitudes, but in most of the population it indicates disease. Genetic tests may also be performed to investigate genes and DNA.
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What treatments are available for people with polycythemia?
Treatments for polycythemia and polycythemia vera aim to lower red blood cell volume. Phlebotomy, the removal of blood from a vein until the cellular level is near normal and symptoms subside, is a common method. In some instances, medications or radiation therapy, such as radioactive phosphorus, may be needed to curb overactive marrow cells, but these are avoided when possible due to potential complications. Treating polycythemia involves addressing the underlying cause.
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polycythemia, abnormal increase in red blood cells (erythrocytes) and hemoglobin in the circulation, a situation that results in thickened blood, retarded flow, and an increased danger of clot formation within the circulatory system. The condition often results in an increase in the volume of packed red cells upon hematocrit analysis. Polycythemia occurs in response to some known stimulus for the production of red cells.
Types of polycythemia
Polycythemia may be relative (e.g., after blood plasma loss), transient (as when a large number of red blood cells suddenly enter the circulation from storage), or absolute (i.e., reflecting an increase in actual mass of red cells in the body). Relative polycythemia may be the consequence of abnormally lowered fluid intake or of marked loss of body fluid, such as occurs in persistent vomiting, severe diarrhea, or copious sweating or when water is caused to shift from the circulation into the tissue. Relative and transient, or secondary, polycythemia disappear when the condition to which they are secondary is eliminated. Absolute polycythemia, when the cause is known, is called erythrocytosis.
Causes of polycythemia
Polycythemia is a response by the body to an increased demand for oxygen. It occurs when hemoglobin is not able to pick up large amounts of oxygen from the lungs (i.e., when it is not “saturated”). This may result from decreased atmospheric pressure, as at high altitudes, or from impaired pulmonary ventilation. The sustained increase in red cells in persons who reside permanently at high altitudes is a direct result of the diminished oxygen pressure in the environment. Chronic pulmonary disease (e.g., emphysema—abnormal distension of the lungs with air) may produce chronic hypoxemia (reduced oxygen tension in the blood) and lead to absolute polycythemia. Extreme obesity also may severely impair pulmonary ventilation and thereby cause absolute polycythemia (pickwickian syndrome).
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Congenital heart disorders that permit shunting of blood from its normal path through the pulmonary circuit, thereby preventing adequate aeration of the blood, can also cause polycythemia, as can a defect in the circulating hemoglobin. The latter defect may be congenital because of an enzymatic or a hemoglobin abnormality, or it may be acquired as the result of the excessive use of coal tar derivatives, such as phenacetin, which convert hemoglobin to pigments incapable of carrying oxygen (methemoglobin, sulfhemoglobin). Lastly, polycythemia can develop in the presence of certain types of tumours and as the result of the action of adrenocortical secretions. Treatment of polycythemia due to any of these causes involves the correction or alleviation of the primary abnormality.
Polycythemia vera
Polycythemia differs from a disease called polycythemia vera (erythremia, or primary polycythemia), in which excess red blood cells occur without known cause. In polycythemia vera there is usually an increase in other blood elements as well; for example, the number of red cells and often also the numbers of white blood cells (leukocytes) and platelets (thrombocytes) are increased, and the spleen usually is enlarged. In this disease the stem cell precursor of the bone marrow cells produces excessive progeny. Persons with polycythemia vera have an exceptionally ruddy complexion, with red discoloration of the face and sometimes the extremities, and may have headache, dizziness, difficulty in breathing, a feeling of fullness, skin changes (e.g., tendency to bruise), and an enlarged spleen. However, because certain blood-clotting factors are not produced in adequate amounts, hemorrhages may occur from ulcers or minor wounds. Duodenal ulcer and gout occur with increased frequency in persons with polycythemia vera. The disease is relatively common in Jews, affects men more often than women, and usually appears at middle age or later.
Treatment of polycythemia
Treatment for both polycythemia and polycythemia vera is aimed at reducing the volume of red blood cells. One of the simplest methods is to remove the blood, one pint at a time, from a vein until the cellular level approaches normal and the symptoms disappear. Occasionally it may be necessary to use drugs or radiation therapy, in the form of radioactive phosphorus, to restrain the overactivity of the marrow cells. These treatments must be avoided when possible, however, because of their potential complications.