One basic diagnostic tool used by physicians to help monitor a patient's health is a complete blood count (CBC), a blood test that measures the number of red and white blood cells, as well as platelets. Platelets, also known as thrombocytes, are the smallest component produced by the bone marrow, the tissue found inside the bones where all blood cells are produced. Platelets stick together, or aggregate, and form clots in order to stop bleeding. Elevated platelets can indicate one of two disorders — reactive thrombocytosis or essential thrombocythemia.

If a patient has too few platelets, he or she risks excessive bleeding and bruising. If a patient has elevated platelets, he or she may form clots too easily and block blood flow, causing life threatening problems that may include heart attack, stroke, or pulmonary embolism. The number of platelets in a healthy patient ranges from 150,000 to 400,000 per microliter. When the results of a platelet count exceed this amount, the patient is considered to have elevated platelets.

Reactive thrombocytosis, also known as secondary thrombocytosis, is the most common cause of elevated platelets. This disorder occurs in response to other diseases or conditions. These may include chronic inflammatory conditions, acute infection, some anemias, cancer, and other blood disorders. In most cases, the effects of reactive thrombocytosis are mild and resolve when the underlying disease or condition is adequately treated.

A more serious cause of elevated platelets is essential thrombocythemia (ET), also known as primary thrombocythemia. ET is a disease that originates in the bone marrow, causing an overproduction of megakaryocytes, the large cells that break into the fragments that form platelets. The platelets produced by this faulty mechanism do not always function in a normal manner. Whereas the patient diagnosed with reactive thrombocytosis must watch for problems caused by excess clotting, the elevated platelets in a patient with ET may cause abnormal clotting or bleeding. Therefore, it is important for a patient with elevated platelets to receive the correct diagnosis.

When a patient presents with an elevated platelet count, the physician should first determine if the condition is secondary to any other disease or condition, making the diagnosis "reactive thrombocytosis." These patients generally have platelet counts of less than 1,000,000 per microliter. If no other precipitating condition can be found, a bone marrow biopsy is performed to look for evidence of ET. Platelet aggregation studies to evaluate the ability of the platelets to clot are also ordered to identify abnormalities that may suggest a diagnosis of ET.

Once the physician has received all results of testing, a diagnosis of either reactive thrombocytosis or essential thrombocythemia is made and the patient receives appropriate treatment. Treatment for reactive thrombocytosis is simply achieved through addressing the related disease. Treatment of ET may consist of aspirin therapy and medications to reduce platelets. ET is not curable, and the disease predisposes a small percentage of patients to develop acute leukemia and fibrosis of the bone marrow.