Evaluation of Antiplatelet Autoantibody
**Note! If you have trouble understanding this section, refer to the sections on Definition of General Terms and on Cause and Pathophysiology.
The diagnosis of chronic ITP can be made without measuring antiplatelet antibodies. The measurement of antiplatelet antibody is primarily of research interest, although it may be useful in cases where there is confusion about the diagnosis. Either autoantibody bound to the patient's platelets (platelet-associated autoantibody, the most sensitive test) or autoantibody circulating in the plasma (plasma autoantibody) can be measured.
Specific tests. There are two excellent tests to measure platelet-associated and plasma autoantibody: the immunobead assay and the monoclonal antibody-specific immobilization of platelet antigens (MAIPA) assay. Both tests measure autoantibody against antigens on specific platelet glycoprotein complexes (a complex is a combination of two or more proteins), usually glycoprotein IIb/IIIa (a combination of platelet glycoprotein IIb and platelet glycoprotein IIIa) and glycoprotein Ib/IX (a combination of platelet glycoprotein Ib and platelet glycoprotein IX). These tests give positive results in about 60% of ITP patients and are not positive in patients with other causes of thrombocytopenia. Unfortunately, these specific tests are done primarily in research laboratories and are unavailable in most clinical laboratories.
Non-specific test. The test used in most clinical laboratories measures platelet-associated IgG. This detects any IgG antibody (both specific antiplatelet antibody and any other IgG antibody associated with the platelet). Although this test gives positive results in about 90% of chronic ITP patients, it may also be falsely positive in patients who have other causes of a low platelet count that are not due to antiplatelet autoantibody. Therefore, this test does not specifically measure antiplatelet antibody and most people in the field do not recommend it.
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