Primary immune thrombocytopenia, also known as idiopathic or immune thrombocytopenic purpura (ITP), is an acquired thrombocytopenia possibly caused by autoantibodies directed against platelet antigens. It is defined as isolated thrombocytopenia (platelet count < 100,000/µL) without anemia, leukopenia, or any other cause of thrombocytopenia; thus, it is a diagnosis of exclusion. ITP is considered a common acquired bleeding disorder. It is often a chronic condition in adults, and prevalence therefore exceeds incidence. Prevalence is estimated to be approximately eight cases per 100,000 children and 12 cases per 100,000 adults in the United States. Distinguishing ITP from other causes of thrombocytopenia that may present in a similar fashion is important, as is determining whether ITP is primary or secondary to an underlying medical condition, as this can affect management.
Primary ITP accounts for most cases, and a presumptive diagnosis of primary ITP is made when the history, physical examination, and laboratory testing excludes all other causes of thrombocytopenia. Secondary ITP is diagnosed in a patient with ITP and an underlying condition that can mediate it, such as HIV infection, hepatitis C virus (HCV) infection, systemic lupus erythematosus (SLE), and chronic lymphocytic leukemia. Drugs can also cause secondary ITP or drug-induced immune thrombocytopenia. Commonly implicated drugs include beta-lactam antibiotics, trimethoprim-sulfamethoxazole, phenytoin, quinine, quinidine, and carbamazepine.
The pathogenesis of ITP is still incompletely understood. The underlying mechanism for the reduced platelet lifespan may be secondary to the presence of antibody-mediated destruction of platelets.[1,2] Specifically, the production of immunoglobulin G autoantibodies by B cells directed against platelet membrane glycoproteins (such as IIb/IIIa) may be responsible.[1,2,3] In some patients an inciting event is present, such as a preceding viral infection or, less commonly, a bacterial infection that triggers the production of these antibodies. These antibodies then cross-react with normal platelet antigens, causing destruction of platelets.
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