Platelets are blood cell fragments involved in clotting. According to the National Heart Lung and Blood Institute, a normal platelet level is between 150,000 to 450,000 platelets per microliter of blood. High platelet levels in the blood are referred to as primary thrombocythemia or secondary thrombocytosis.
Primary thrombocythemia isn't common and occurs when faulty cells in the bone marrow increase platelet production. The platelets of someone with primary thrombocythemia may rise higher than 1 million platelets per microliter of blood. Secondary thrombocytosis is more common, and the platelet count is generally less than 1 million platelets per microliter of blood.
The cause of primary thrombocythemia isn't known, but a rare form of this condition is inherited. Secondary thrombocytosis is caused by iron-deficiency anaemia, hemolytic anaemia, surgical removal of the spleen, reactions to medications, serious blood loss, short-lived infection or inflammation, response to physical activity, excessive alcohol use, lack of vitamin B12 or folate, and cancer of the lung, stomach, intestines, breasts, ovaries or lymphatic system. This condition can also occur due to long-lasting infectious or inflammatory conditions such as tuberculosis, inflammatory bowel disease or connective tissue disorder.
Primary thrombocythemia can cause blood clots anywhere in the body and bleeding. A blood clot in the brain can cause headache, dizziness or a stroke. A blood clot in the hands or feet can cause numbness, redness and a burning or throbbing pain. Blood clots can cause additional symptoms such as changes in speech, confusion, unconsciousness, seizures, shortness of breath, nausea and discomfort in one or both arms, jaw, neck or back. In pregnant women, a blood clot in the placenta can cause miscarriage or fetal death. The risk of blood clots increases if you are older than 60 years of age, have a history of blood clots, smoke or have an underlying condition such as diabetes or high blood pressure.
Bleeding due to a platelet disorder usually occurs in people with a platelet count greater than 1 million. Signs and symptoms of bleeding are bruising, nosebleeds, bleeding from the gums or mouth and blood in your stool. People with secondary thrombocytosis usually don't have symptoms and are at a lower risk of blood clots or bleeding, but certain factors such as bed rest or severe disease of the arteries can increase this risk.
Diagnosis of an increased platelet count is based on your medical history and a physical exam to check for underlying conditions as well as the presence of and duration of signs and symptoms. Your physician may order additional diagnostic testing such as blood work, bone marrow tests to check for bone marrow abnormalities, and genetic testing to rule out genetic causes of a high platelet count. Blood tests include a complete blood count to measure the levels of the different blood cells and a blood smear to check the condition of the platelets. Not all tests to assess the cause of a high platelet count are indicated for every person.
Primary thrombocythemia is treated with blood-thinning or platelet-lowering medications and plateletpheresis. Platelet-lowering medications are taken for the rest of your life and are generally indicated if you have a history of blood clots or bleeding, high blood cholesterol, high blood pressure, diabetes, age greater than 60 and a platelet count over 1 million. Plateletpheresis is an emergency treatment used to rapidly lower the platelet count. Treatment of secondary thrombocytosis is aimed at controlling the underlying cause. Your physician will discuss with you which treatments will be most effective in your situation.