Idiopathic thrombocytopenic purpura laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Laboratory Findings

The diagnosis of ITP is a diagnosis of exclusion. First, one has to make sure that there are no other blood abnormalities except for low platelet count and no physical signs except for signs of bleeding. Then, the secondary causes (usually 5-10% of suspected ITP cases) should be excluded. Secondary causes could be leukemia, medications (e.g. quinine, heparin), lupus erythematosus, cirrhosis, HIV, hepatitis C, congenital causes, antiphospholipid syndrome, von Willebrand factor deficiency and others.

Despite the destruction of platelets by splenic macrophages, the spleen is normally not enlarged. In fact, an enlarged spleen should lead a clinician to investigate other possible causes for the thrombocytopenia.

Bleeding time is prolonged in ITP patients; however, the use of bleeding time in diagnosis is discouraged by the American Society of Hematology practice guidelines as useless. For example the BMJ review of the basics of hematology states: "The bleeding time may or may not be prolonged in congenital or acquired platelet dysfunction, and therefore a normal bleeding time does not exclude these conditions."

Platelet-associated antibody (IgG), which was the standard test of past years, is now not considered mandatory to diagnose ITP. Test for platelet antibody are not helpful as both their sensitivity and specificity are limited. The blood analysis for the antiplatelet antibodies is a matter of clinician's preference, as there is a disagreement whether the 80% specificity of this test is sufficient. In conditions associated with bone marrow failure (aplastic anemia) thrombopoietin (TPO)levels are high whereas in ITP thrombopoietin levels are low. Thus TPO could distinguish between decreased platelets due to bone marrow failure or increased due to their destruction. The bone marrow in ITP contains normal or high numbers of megakaryocytes but they may be small or immature (& may have been damaged by antibodies).