How to diagnose Thrombocytopenia ? 

Your doctor will diagnose thrombocytopenia 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 diagnosing and treating blood diseases and conditions.
                
Once thrombocytopenia is diagnosed, your doctor will begin looking for its cause.
                
Medical History
                
Your doctor may ask about factors that can affect your platelets, such as:
                
The medicines you take, including over-the-counter medicines and herbal remedies, and whether you drink beverages that contain quinine. Quinine is a substance often found in tonic water and nutritional health products.
                
Your general eating habits, including the amount of alcohol you normally drink.
                
Your risk for AIDS, including questions about blood transfusions, sexual partners, intravenous (IV) drugs, and exposure to infectious blood or bodily fluids at work.
                
Any family history of low platelet counts.
                
Physical Exam
                
Your doctor will do a physical exam to look for signs and symptoms of bleeding, such as bruises or spots on the skin. He or she will check your abdomen for signs of an enlarged spleen or liver. You also will be checked for signs of infection, such as a fever.
                
Diagnostic Tests
                
Your doctor may recommend one or more of the following tests to help diagnose a low platelet count. For more information about blood tests, go to the Health Topics Blood Tests article.
                
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 thrombocytopenia, the results of this test will show that your platelet count is low.
                
Blood Smear
                
A blood smear is used to check the appearance of your platelets under a microscope. For this test, a small amount of blood is drawn from a blood vessel, usually in your arm.
                
Bone Marrow Tests
                
Bone marrow tests check whether your bone marrow is healthy. Blood cells, including platelets, are made in your bone marrow. The two bone marrow tests are aspiration (as-pih-RA-shun) and biopsy.
                
Bone marrow aspiration might be done to find out why your bone marrow isn't making enough blood cells. 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 sample 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.
                
Other Tests
                
If a bleeding problem is suspected, you may need other blood tests as well. For example, your doctor may recommend PT and PTT tests to see whether your blood is clotting properly.
                
Your doctor also may suggest an ultrasound to check your spleen. An ultrasound uses sound waves to create pictures of your spleen. This will allow your doctor to see whether your spleen is enlarged.