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How ITP Is Diagnosed

What are the tests for ITP?

ITP cannot be accurately diagnosed by any one test. When someone has a low platelet count, a doctor runs tests to exclude other conditions that might cause platelet counts to drop. ITP is only diagnosed when all other causes have been ruled out. This is called "diagnosis by exclusion."1,2

Common tests include:

  • Complete blood count (CBC): The most frequently performed test is a complete blood count, which tells a doctor the number of each kind of blood cell (red cells, white cells, and platelets) in the sample. Lower platelet levels indicate more severe forms of ITP.3,4
  • Blood smear: Blood is checked under a microscope and a doctor (usually a pathologist) looks for abnormal cells. In most cases, abnormal cells in the blood would indicate a diagnosis different from ITP.5
  • Bone marrow examination: A sample of the bone marrow is drawn and examined under a microscope. The bone marrow of someone with ITP will show normal (or even increased) numbers of megakaryocytes—the cells that produce platelets. If all the other cells in the bone marrow are normal in number and appearance and there is a low platelet count in the blood, a diagnosis of ITP is suggested.6 (This procedure is recommended for patients who are over age 60 and those for whom splenectomy is being considered.)5


  1. Cines DB, Bussel JB. How I treat idiopathic thrombocytopenic purpura. Blood. 2005;106(7):2244-2251.
  2. Cines DB, Blanchette VS. Immune thrombocytopenic purpura. N Engl J Med. 2002;346(13):995-1008.
  3. Merck Manual Home Edition. Diagnosis; Symptoms and Diagnosis of Blood Disorders. http://www.merck.com/mmhe/sec14/ch170/ch170c.html. Accessed April 14, 2009.
  4. Merck Manual Home Edition. Thrombocytopenia (ITP, TTP). http://www.merck.com/mmhe/sec14/ch173/ch173d.html. Accessed April 14, 2009.
  5. George JN, Woolf SH, Raskob GE, et al. Idiopathic thrombocytopenic purpura: a practice guideline developed by explicit methods for the American Society of Hematology. Blood. 1996;88(1):3-40.
  6. Cines DB, McMillan R. Management of adult idiopathic thrombocytopenic purpura. Annu Rev Med. 2005;56:425-442.