Thrombocytosis involves having an elevated level of platelets in your blood. Platelets are blood cells that form clots to stop bleeding. There are two main types of thrombocytosis: essential thrombocythemia and reactive thrombocytosis. Usually, thrombocytosis isn’t serious. But too many platelets can cause complications like a stroke, heart attack or a clot in your blood vessels.


What is thrombocytosis?

Thrombocytosis (pronounced “throm-boe-sie-TOE-sis”) is having too many platelets in your blood. Platelets are blood cells that stop bleeding by sticking together to form a clot. Too many platelets, however, can cause your blood to become too sticky.

In severe cases, thrombocytosis can cause dangerous clots in your blood vessels, increasing your risk of a stroke or heart attack.

Your experience of thrombocytosis, including its seriousness and whether you need treatment, depends on its cause.

What are the types of thrombocytosis?

There are two types of thrombocytosis: essential thrombocythemia and reactive thrombocytosis.

Essential thrombocythemia (primary thrombocytosis)

Essential thrombocythemia (ET), or primary thrombocytosis, is a rare blood disorder in which your bone marrow makes too many platelets. Your bone marrow makes most of your body’s blood cells, including platelets. With essential thrombocythemia, blood cell production goes wrong, causing you to have abnormal and excess platelets.

ET is also the most common myeloproliferative neoplasm (MPN).

Reactive thrombocytosis (secondary thrombocytosis)

Reactive, or secondary, thrombocytosis happens when your platelets increase in response to something else — a condition, an injury, an infection, surgery, absent spleen, etc. With reactive thrombocytosis, you have high platelets because your body “reacts” to an underlying cause. Reactive thrombocytosis is often temporary.

Who gets thrombocytosis?

Most people with thrombocytosis are diagnosed at about age 60. Women and people assigned female at birth are diagnosed with essential thrombocythemia twice as much as men and people assigned male at birth.


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Symptoms and Causes

What causes thrombocytosis?

Essential thrombocythemia (ET) and reactive thrombocytosis have different causes.

Essential thrombocythemia

Essential thrombocythemia is an acquired genetic condition, meaning you develop it over time. With ET, some of the genes that play a role in platelet production change, or mutate. As a result, your bone marrow makes too many platelets.

More than half the people with essential thrombocythemia have a gene mutation called JAK2. Other common mutations affect the CALR or MPL gene.

Scientists aren’t sure still what causes the gene mutations, but research is ongoing.

Reactive thrombocytosis

Reactive thrombocytosis happens when your body reacts to various conditions and situations by overproducing platelets. Or your body may not destroy platelets your body no longer needs at the normal rate, causing too many to accumulate.

Causes of reactive thrombocytosis include:

There are many more causes of reactive thrombocytosis.

What are the symptoms of thrombocytosis?

Most people with high platelets don’t have symptoms, at least at first. If you have reactive thrombocytosis, you may experience symptoms associated with the condition causing it.

When symptoms occur, they often relate to abnormal bleeding and clotting in your body. Symptoms include:

  • Headache.
  • Bruising easily.
  • Feeling weak, lightheaded or dizzy.
  • Bleeding from your nose, mouth and gums.
  • Bleeding in your stomach or intestinal tract.

Some people with essential thrombocythemia develop erythromelalgia, a condition that causes pain, swelling and redness in your hands and feet. You may also notice numbness and tingling.

In severe cases, thrombocytosis can cause unusual blood clots in your abdomen (Budd-Chari syndrome) and increase your risk of a stroke or heart attack.


Diagnosis and Tests

How is thrombocytosis diagnosed?

As thrombocytosis doesn’t typically cause symptoms, the first sign is often a high platelet count that shows up during routine blood work (complete blood count).

Thrombocytosis involves having more than 450,000 platelets per microliter of blood. If you have elevated levels, your healthcare provider will likely order a follow-up blood test a few weeks later to see if your levels remain high. Levels that are momentarily high but then return to normal aren’t usually concerning. Levels that remain elevated may signal an underlying condition.

Identifying the underlying condition (such as iron deficiency anemia, cancer or infection) that’s raising your levels helps healthcare providers diagnose and manage reactive thrombocytosis. If your provider can’t find a secondary cause, they’ll run tests to see if you have essential thrombocythemia.

Tests may include:

Management and Treatment

How is thrombocytosis treated?

If you don’t have symptoms, you may only need routine checkups.

Secondary forms of thrombocytosis rarely require treatment. Usually, levels return to normal after the condition causing high platelets (injury, infection, response to surgery, etc.) resolves. If you do have symptoms, your healthcare provider will treat the underlying cause.

Your provider may recommend a few treatment options if you have essential thrombocythemia and are experiencing symptoms:

  • Taking a daily low-dose aspirin: You can take aspirin to prevent blood clots. The low dose doesn’t usually cause stomach upset or bleeding. Be sure to check with your provider before taking aspirin regularly.
  • Taking medicines that lower platelet levels: Medications such as hydroxyurea(Droxia®, Hydrea®, Siklos®, Mylocel®) and anagrelide (Agrilyn®) suppress platelet production in your bone marrow. Interferon alfa (Multiferon®) prevents abnormal platelets from dividing and multiplying.
  • Having a procedure to remove your platelets: If your platelets are so high that they’re putting you at risk of life-threatening blood clots, your provider may recommend plateletpheresis. This procedure uses a machine to filter excess platelets from your blood. Most causes of thrombocytosis don’t require plateletpheresis.


Can thrombocytosis be cured?

There’s no cure for essential thrombocytosis, but your healthcare provider can help you manage the condition to reduce your risk of clots. High platelet levels associated with secondary causes usually return to normal once the underlying condition resolves.


Can thrombocytosis be prevented?

Most conditions that cause thrombocytosis, including essential thrombocythemia, aren’t preventable.

Outlook / Prognosis

Is thrombocytosis life-threatening?

High platelets aren’t life-threatening, but the complications that can result from the condition — blood clots or severe bleeding — can be.

The most common causes of thrombocytosis are short-lived and don’t put you at risk of serious blood clots. The risk is greater with essential thrombocytosis. Still, your healthcare provider will monitor you closely, prescribe medications and perform necessary procedures to prevent harmful blood clots.

What is the outlook for someone with thrombocytosis?

Reactive thrombocytosis gets better when the underlying problem resolves. Although your platelet count is elevated for a short time (or even long-term after splenectomy), secondary thrombocytosis doesn’t typically lead to abnormal blood clotting.

Essential thrombocythemia (ET), or primary thrombocytosis, can cause serious bleeding or clotting complications. Taking medicine that keeps your platelet levels normal can help prevent this. After many years of having the disease, however, bone marrow fibrosis (scarring) can develop. A small percentage of people with essential thrombocythemia develop leukemia.

Living With

When should I see my healthcare provider?

Follow your provider’s instructions on how often you should be having checkups and getting blood tests to check your platelet count.

When should I go to ER?

Call 911 immediately if you’re having symptoms of a stroke or heart attack.

What questions should I ask my doctor?

Questions to ask include:

  • Should I be concerned that my platelet levels are high?
  • Will I need follow-up tests to check my platelets?
  • What tests will I need to determine what’s causing my thrombocytosis?
  • What treatments do you recommend?
  • What lifestyle changes can I make to manage my condition?

A note from Cleveland Clinic

Don’t be alarmed if your blood work results show high platelet levels. Your platelets may be elevated for various reasons. Many causes don’t require treatment. If your levels remain high and you’re experiencing symptoms, your healthcare provider will work to determine the cause. Careful monitoring and medications can usually prevent the most concerning complications associated with thrombocytosis.

Medically Reviewed

Last reviewed on 09/12/2022.

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