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      Thrombocythemia and Thrombocytosis
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What Are Thrombocythemia and Thrombocytosis?

Thrombocythemia (THROM-bo-si-THE-me-ah) and thrombocytosis (THROM-bo-si-TO-sis) are conditions in which your blood has a high number of blood cell fragments called platelets (PLATE-lets).

Platelets are made in your bone marrow along with other kinds of blood cells. They travel through your blood vessels and stick together (clot) to stop any bleeding that could happen if a blood vessel is damaged. Platelets also are called thrombocytes (THROM-bo-sites), because a clot also is called a thrombus.

A normal platelet count ranges from 150,000 to 450,000 platelets per microliter of blood.


The term "thrombocythemia" is preferred when the cause of the high platelet count isn't known. The condition is then called primary or essential thrombocythemia.

This condition occurs when faulty cells in the bone marrow make too many platelets. Bone marrow is the sponge-like tissue inside the bones. It contains stem cells that develop into red blood cells, white blood cells, or platelets. What causes the bone marrow to make too many platelets often isn't known.

With primary thrombocythemia, a high platelet count may occur alone or with other blood cell disorders. The platelet count can be as low as 500,000 platelets per microliter of blood or higher than 1 million platelets per microliter of blood. This condition isn't common.

When another disease or condition causes a high platelet count, the term "thrombocytosis" is preferred. This condition often is called secondary or reactive thrombocytosis.

In this condition, the platelet count usually is less than 1 million platelets per microliter of blood. Secondary thrombocytosis is more common than primary thrombocythemia.

Most people who have a high platelet count don't have signs or symptoms. Rarely, serious or life-threatening symptoms can develop, such as blood clots and bleeding. These symptoms mostly occur in people who have primary thrombocythemia.


People who have primary thrombocythemia but no signs or symptoms don't need treatment, as long as the condition remains stable. Other people who have this condition may need medicines or procedures to treat it. Most people who have primary thrombocythemia will live a normal life span.

Treatment and outlook for secondary thrombocytosis depend on its underlying cause.

February 2008

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