Tests and diagnosis

By Mayo Clinic Staff

Doctors usually diagnose idiopathic thrombocytopenic purpura by excluding other possible causes of bleeding and a low platelet count, such as an underlying illness or medications you may be taking. If no other underlying problem is causing your signs and symptoms, then a diagnosis of ITP may be made.

To diagnose ITP, your doctor may need a:

  • Physical exam, including a complete medical history. Your doctor will look for signs of bleeding under your skin, and will ask you about previous illnesses you've had and the types of medications and supplements that you've recently taken.
  • Complete blood count. This common blood test is used to determine the number of white and red blood cells and platelets in a sample of your blood. With ITP, white and red blood cell counts are usually normal, but the platelet count is low.
  • Blood smear. A sample of your blood is placed on a slide and observed under a microscope. This test is often used to confirm the number of platelets observed in a complete blood count.
  • Bone marrow examination. Another test that may help identify the cause of a low platelet count is a bone marrow exam. Platelets are produced in your bone marrow — soft, spongy tissue in the center of your large bones. In some cases, a sample of solid bone marrow is removed in a procedure called a bone marrow biopsy. Or, you may have a bone marrow aspiration, which removes some of the liquid portion of your marrow. In many cases, both procedures are performed at the same time (bone marrow exam). Both the liquid and solid bone marrow samples are frequently taken from the same place on the back of one of your hipbones. A needle is inserted into the bone through an incision.

If you have ITP, your bone marrow will be normal because your low platelet count is caused by the destruction of platelets in your bloodstream and spleen — not by a problem with the bone marrow.

Dec. 10, 2014