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ITP is usually diagnosed by a blood test showing that only the platelet count is low, and the platelets, red blood cells and white blood cells all look normal. A bone marrow biopsy may be taken at a later stage if the ITP continues, in which a small sample of bone marrow will be taken under local anaesthetic and examined under the microscope. Additional blood tests may be taken at this time to exclude rare clotting or immune diseases that can mimic ITP. If the bone marrow looks normal, with the usual or higher number of platelet parent cells (megakaryocytes) and other blood tests are normal then the doctor will diagnose chronic ITP.