Your doctor will diagnose thrombocythemia or thrombocytosis based on your medical history, a physical exam, and test results. A hematologist also may be involved in your care. This is a doctor who specializes in blood diseases and conditions.
                
Medical History
                
Your doctor may ask you about factors that can affect your platelets, such as:
                
Any medical procedures or blood transfusions you've had
                
Any recent infections or vaccines you've had
                
The medicines you take, including over-the-counter medicines
                
Your general eating habits, including the amount of alcohol you normally drink
                
Any family history of high platelet counts
                
Physical Exam
                
Your doctor will do a physical exam to look for signs and symptoms of blood clots and bleeding. He or she also will check for signs of conditions that can cause secondary thrombocytosis, such as an infection.
                
Primary thrombocythemia is diagnosed only after all possible causes of a high platelet count are ruled out. For example, your doctor may recommend tests to check for early, undiagnosed cancer. If another disease, condition, or factor is causing a high platelet count, the diagnosis is secondary thrombocytosis.
                
Diagnostic Tests
                
Your doctor may recommend one or more of the following tests to help diagnose a high platelet count.
                
Complete Blood Count
                
A complete blood count (CBC) measures the levels of red blood cells, white blood cells, and platelets in your blood. For this test, a small amount of blood is drawn from a blood vessel, usually in your arm.
                
If you have thrombocythemia or thrombocytosis, the CBC results will show that your platelet count is high.
                
Blood Smear
                
A blood smear is used to check the condition of your platelets. For this test, a small amount of blood is drawn from a blood vessel, usually in your arm. Some of your blood is put on a glass slide. A microscope is then used to look at your platelets.
                
Bone Marrow Tests
                
Bone marrow tests check whether your bone marrow is healthy. Blood cells, including platelets, are made in the bone marrow. The two bone marrow tests are aspiration (as-pih-RA-shun) and biopsy.
                
Bone marrow aspiration might be done to find out whether your bone marrow is making too many platelets. For this test, your doctor removes a sample of fluid bone marrow through a needle. He or she examines the sample under a microscope to check for faulty cells.
                
A bone marrow biopsy often is done right after an aspiration. For this test, your doctor removes a small amount of bone marrow tissue through a needle. He or she examines the tissue to check the number and types of cells in the bone marrow.
                
With thrombocythemia and thrombocytosis, the bone marrow has a higher than normal number of the very large cells that make platelets.
                
Other Tests
                
Your doctor may recommend other blood tests to look for genetic factors that can cause a high platelet count.