Diagnosis

To diagnose idiopathic thrombocytopenic purpura, your doctor will try to exclude other possible causes of bleeding and a low platelet count, such as an underlying illness or medications you or your child may be taking.

Your doctor will also ask you about your or your child's medical history, perform a physical exam and run one or more of the following tests:

  • Complete blood count (CBC). This common blood test is used to determine the number of blood cells, including platelets, in a sample of blood. With ITP, white and red blood cell counts are usually normal, but the platelet count is low.
  • Blood smear. This test is often used to confirm the number of platelets observed in a complete blood count. A sample of blood is placed on a slide and observed under a microscope.
  • Bone marrow exam. This test may be used to help identify the cause of a low platelet count, though the American Society of Hematology doesn't recommend this test for children with ITP.

  • Platelets are produced in the bone marrow — soft, spongy tissue in the center of large bones. In some cases, a sample of bone tissue and the enclosed marrow is removed in a procedure called a bone marrow biopsy. Or your doctor may do a bone marrow aspiration, which removes some of the liquid portion of the marrow. In many cases, both procedures are performed at the same time (bone marrow exam).

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