How Is Idiopathic Thrombocytopenic Purpura Diagnosed?

To diagnose idiopathic thrombocytopenic purpura (ITP), your doctor must first be certain that your low platelet count isn’t caused by other conditions, such as HIV infection or lupus, or by medicines (for example, chemotherapy drugs or aspirin). Your doctor will ask about your medical history, do a physical exam, and order blood tests.

Your medical history includes information about:

• Your signs and symptoms of bleeding
• Illnesses you have that could lower your platelet count or cause bleeding
• Medicines or any other over-the-counter supplements or remedies you take that could cause bleeding or lower your platelet count

Your doctor will give you a physical exam and look for signs of bleeding and infection.

He or she also will order blood tests to measure the platelet count in your blood. These tests usually include:

• A complete blood count. This test shows the numbers of different kinds of blood cells, including platelets, in a small sample of your blood. In ITP, the red and white blood cell counts are normal.
• A blood smear. During this test, some of your blood is put on a slide. A microscope is then used to look at your platelets and other blood cells.
• Some laboratories may be able to test for the antibodies that attack platelets.

If blood tests show that you have a low number of platelets, your doctor may need to order more tests to confirm a diagnosis of ITP. For example, bone marrow tests may be used to see if the large cells that make platelets look normal. (These large cells are called megakaryocytes.)

Some people with mild ITP have few or no signs of bleeding. In that case, they might be diagnosed only after a blood test done for another reason shows that they have a low number of platelets.