The Serological Investigation of Patients with Autoimmune Thrombocytopenia

Techniques for measuring the plasma factor responsible for idiopathic thrombocytopenic purpura (ITP) have been sought for some 40 years, but unlike red cell serological techniques, platelet antibody testing has proved difficult and, at times, generated controversial results. The difficulty in measuring platelet antibodies probably reflects the intrinsic nature of the platelet membrane--to act as a sponge and nonspecifically bind plasma proteins. There have now been three different general types of techniques used to measure platelet autoantibodies: phase I assays measuring a platelet-dependent endpoint after incubating test serum and control platelets. These assays lack sufficient sensitivity and specificity to be useful. Except for special instances (the diagnosis of heparin-induced thrombocytopenia), they are no longer used. Phase II assays measure IgG on the surface of platelet or within the platelets following lysis. The immunoglobulin detected is termed platelet associated IgG. PAIgG assays are falling out of favour because they lack the sensitivity and specificity to be useful diagnostic tools. Most recently, the ability to isolate individual platelet glycoproteins and measure the binding of IgG to these glycoproteins has dramatically increased our understanding of ITP. These Phase III assays may prove useful diagnostic tools for the investigation and classification of immune thrombocytopenic disorders.