Thrombocytosis (throm-boe-sie-TOE-sis) is a disorder in which your body produces too many platelets (thrombocytes), which play an important role in blood clotting. The disorder is called reactive thrombocytosis when it's caused by an underlying condition, such as an infection.
Thrombocytosis may also be caused by a blood and bone marrow disease. When caused by a bone marrow disorder, thrombocytosis is called autonomous, primary or essential thrombocytosis or essential thrombocythemia.
Your doctor may detect thrombocytosis in routine blood test results that show a high platelet level. If your blood test indicates thrombocytosis, it's important for your doctor to determine whether it's reactive thrombocytosis or if you have thrombocythemia, which is more likely to cause blood clots.