Overview

Platelets are blood particles produced in the bone marrow that play an important role in the process of forming blood clots. Thrombocytosis (throm-boe-sie-TOE-sis) is a disorder in which your body produces too many platelets.

It's called reactive thrombocytosis or secondary thrombocytosis when the cause is an underlying condition, such as an infection.

Less commonly, when thrombocytosis has no apparent underlying condition as a cause, the disorder is called primary thrombocythemia or essential thrombocythemia. This is a blood and bone marrow disease.

Your doctor might detect thrombocytosis in a routine blood test result that shows a high platelet level. If your blood test indicates thrombocytosis, it's important to determine whether it's reactive thrombocytosis or essential thrombocythemia to know how to manage the condition.