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.

How Are Thrombocythemia and Thrombocytosis Diagnosed?

Your doctor will diagnose thrombocythemia or thrombocytosis based on your medical history, a physical exam, and test results. A hematologist also may be involved in your care. This is a doctor who treats people who have blood diseases.

Medical History

Your doctor may ask you about factors that can affect your platelets, such as:

  • Any medical procedures or blood transfusions you've had
  • Any recent infections or vaccinations you've had
  • The medicines you take, including over-the-counter medicines
  • Your general eating habits, including the amount of alcohol you normally drink
  • Any family history of high platelet counts

Physical Exam

Your doctor will do a physical exam to look for signs and symptoms of bleeding and blood clots. He or she also will check for signs of conditions that can cause secondary thrombocytosis, such as infection.

Primary thrombocythemia is diagnosed only after all other possible causes of a high platelet count are ruled out. For example, your doctor may order tests to check for early, undiagnosed cancer. If another disease, condition, or factor is causing a high platelet count, the diagnosis is secondary thrombocytosis.

Diagnostic Tests

Your doctor may order one or more of the following tests to help diagnose a high platelet count.

Complete Blood Count

A complete blood count (CBC) measures the levels of red blood cells, white blood cells, and platelets in your blood. For this test, a small amount of blood is drawn from a blood vessel, usually in your arm.

If you have thrombocythemia or thrombocytosis, the test results will show that your platelet count is high.

Blood Smear

A blood smear is used to check the condition of your platelets. For this test, a small amount of blood is drawn from a blood vessel, usually in your arm. Your doctor looks at the blood sample under a microscope.

Bone Marrow Tests

Bone marrow tests check whether your bone marrow is healthy. Blood cells, including platelets, are made in bone marrow. The two bone marrow tests are aspiration (as-pi-RA-shun) and biopsy.

Bone marrow aspiration may be done to find out whether your bone marrow is making too many platelets. For this test, your doctor removes a small amount of fluid bone marrow through a needle. He or she examines the sample under a microscope to check for faulty cells.

A bone marrow biopsy often is done right after an aspiration. For this test, your doctor removes a small amount of bone marrow tissue through a needle. He or she examines the tissue to check the number and types of cells in the bone marrow. With thrombocythemia and thrombocytosis, the bone marrow has a higher than normal number of the very large cells that make platelets.

Other Tests

Your doctor may order other blood tests to look for genetic factors that can cause a high platelet count.

What Causes Thrombocythemia and Thrombocytosis?

Primary Thrombocythemia

In this condition, faulty stem cells in the bone marrow make too many platelets. What causes this to happen usually isn't known. When this process occurs without affecting other blood cells, it's called essential thrombocythemia.

A rare form of thrombocythemia is inherited. ("Inherited" means the condition is passed from parents to children.) In some cases, a genetic mutation may cause the condition.

In primary thrombocythemia, the platelets aren't normal. They may form blood clots, or, surprisingly, cause bleeding when they don't work properly.

Bleeding also can occur because of a condition that develops called von Willebrand disease. This condition affects the blood clotting process.

After many years, scarring of the bone marrow can occur.

Secondary Thrombocytosis

This condition occurs when another disease, condition, or outside factor causes the platelet count to rise. For example, 35 percent of people who have high platelet counts also have cancer—mostly lung, gastrointestinal, breast, ovarian, and lymphoma. Sometimes a high platelet count is the first sign of cancer.

Unlike primary thrombocythemia, the platelets in secondary thrombocytosis usually are normal.

Conditions or factors that can cause a high platelet count are:

  • Iron-deficiency anemia (uh-NEE-me-uh)
  • Hemolytic (HEE-moh-lit-ick) anemia
  • Absence of a spleen (after surgery to remove this organ)
  • Cancer
  • Inflammatory or infectious diseases such as connective tissue disorders, inflammatory bowel disease, and tuberculosis
  • Reactions to medicines

Some conditions can lead to a high platelet count that lasts for only a short time. These include:

  • Recovery from serious loss of blood
  • Recovery from a very low platelet count caused by excessive alcohol use and lack of vitamin B12 or folate
  • Acute infection or inflammation
  • Response to physical activity


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