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.
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.
There are two types of thrombocytosis: essential thrombocythemia and reactive 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, 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.
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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Essential thrombocythemia (ET) and reactive thrombocytosis have different causes.
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 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.
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:
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.
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:
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:
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.
Most conditions that cause thrombocytosis, including essential thrombocythemia, aren’t preventable.
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.
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.
Follow your provider’s instructions on how often you should be having checkups and getting blood tests to check your platelet count.
Questions to ask include:
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.
Last reviewed by a Cleveland Clinic medical professional on 09/12/2022.
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