A condition in which the body overproduces the blood cells that help clotting.

Thrombocytosis (throm-boe-sie-TOE-sis) happens when the body produces too many blood cells that help clotting, called platelets. It can be caused by an underlying condition, such as an infection. Less commonly, it is caused by a blood and bone marrow disease. A high platelet level may be detected in a routine blood test known as a complete blood count.

People with a high platelet level often don't have symptoms. When symptoms occur, they may include headache, confusion, chest pain, shortness of breath, weakness, and burning pain in the hands or feet. There also may be symptoms caused by the underlying condition.

Treatment is aimed at the underlying cause. Sometimes, a high platelet count resolves on its own. If not, a health care professional may prescribe platelet-lowering drugs.