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.