What is thrombocytosis?

Thrombocytosis refers to having too many platelets in your blood. Platelets are blood cells in plasma that stop bleeding by sticking together to form a clot. Too many platelets can lead to certain conditions, including stroke, heart attack or a clot in the blood vessels. There are two types of thrombocytosis: primary and secondary.

Primary thrombocytosis is a disease in which abnormal cells in the bone marrow cause an increase in platelets. It is also called essential thrombocythemia (or ET). The cause is unknown. It isn’t considered an inherited (genetic) condition even though certain gene mutations have been found in the blood or bone marrow.

Secondary, or reactive, thrombocytosis is caused by another condition the patient may be suffering from, such as:

  • Anemia due to iron deficiency.
  • Cancer.
  • Inflammation or infection,
  • Surgery, especially splenectomy (removal of the spleen).

Who gets thrombocytosis?

The condition is found most often in older people. In fact, most people with the condition are diagnosed at about age 60.

Symptoms and Causes

What are the symptoms of thrombocytosis?

Most people with high platelet counts don’t have symptoms, at least at the beginning.

If you do have symptoms, they can include:

  • Skin bruising.
  • Bleeding from places like the nose, mouth and gums.
  • Bleeding in the stomach or intestinal tract..

Abnormal blood clotting can also occur, leading to stroke, heart attack and unusual clots in the blood vessels of the abdomen.

Some patients with essential thrombocythemia develop erythromelalgia, a condition that causes pain, swelling and redness of your hands and feet. Numbness and tingling also occur.

Diagnosis and Tests

How is thrombocytosis diagnosed?

Finding the underlying condition (such as iron deficiency anemia, cancer or infection) can aid in the diagnosis and management of thrombocytosis. If no secondary cause is identified, it’s important to rule out essential thrombocythemia.

Your healthcare provider may order a blood test for a specific gene, called JAK2, which is used to diagnose ET. However, it is positive in only about 50% of the cases. Other gene mutations are also tested, but are only positive in a low percentage of people.

Your provider may suggest a bone marrow biopsy to help confirm the diagnosis.

Management and Treatment

How is thrombocytosis treated?

People who have no symptoms may remain stable and only require routine check-ups by their physician. Secondary forms of thrombocytosis rarely require treatment.

For those with symptoms, a few treatment options are available. One is to treat the disease that is causing thrombocytosis. In some cases, you can take aspirin to help prevent blood clots. The low dose used for this purpose does not usually cause stomach upset or bleeding.

In essential thrombocythemia, medications such as hydroxyurea or anagrelide are used to suppress platelet production by the bone marrow. These medications usually have to be taken indefinitely. Treatment with interferon is sometimes necessary but is associated with a greater number of side effects.

Newer agents are now being developed in an effort to suppress the overproduction of platelets. In cases of severe life-threatening thrombocytosis, a procedure called plateletpheresis is performed to immediately lower the platelet count to safer levels. In this procedure, a special instrument is used to remove blood, separate and remove the platelets, and then return the other blood cells to the patient.

Outlook / Prognosis

What is the outlook for someone with thrombocytosis?

Secondary thrombocytosis gets better when the underlying problem causing the high platelet count resolves. This could mean that your infection is treated or you recover from your surgery. Even though the platelet count is elevated for a short time (or even indefinitely after splenectomy), secondary thrombocytosis does not typically lead to abnormal blood clotting.

Primary thrombocytosis, or essential thrombocythemia, can cause serious bleeding or clotting complications. These can usually be avoided by maintaining good control of the platelet count with medications. After many years of having the disease, however, bone marrow fibrosis (scarring) can develop. In a small percentage of patients, essential thrombocythemia can lead to leukemia.

Last reviewed by a Cleveland Clinic medical professional on 03/02/2021.


  • National Heart, Lung, and Blood Institute. Thrombocythemia and Thrombocytosis. ( Accessed 11/18/2021.
  • Genetic Home Reference. Essential Thrombocythemia. ( Accessed 11/18/2021.
  • Genetic and Rare Diseases Information Center. Essential Thrombocythemia. ( Accessed 11/18/2021.

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