[Idiopathic thrombopenic purpura. Diagnostic and prognostic value of Dixon's test]

The levels of immunoglobulin G (PIgG) bound to platelet membrane can be measured by complement lysis inhibition test (Dixon's assay). In normal subjects, this level is usually below 15 X 10(-15)g = IgG/platelet. Similar values have been obtained with different techniques. PIgG values found on normal platelets might be related to clearance of the oldest platelets. PIgG levels are greatly increased in 85 - 100% of patients with idiopathic thrombocytopenic purpura. Purpura patients with normal PIgG levels might have only C3, IgM or IgA bound to their platelets. The Dixon's assay is not specific to idiopathic thrombocytopenic purpura, since it is positive in other types of thrombocytopenia. PIgG's may be related to platelet size, serum hypergammaglobulinaemia or circulating immune complexes. In the case of idiopathic thrombocytopenic purpura, they probably represent antiplatelet antibodies. The results of Dixon's assay in idiopathic thrombocytopenic purpura must be interpreted with caution. They may offer new possible explanations for the thrombocytopenia associated with other diseases.