Idiopathic thrombocytopenic purpura (ITP), also known as primary immune thrombocytopenia, primary immune thrombocytopenic purpura or autoimmune thrombocytopenic purpura, is defined as isolated low platelet count (thrombocytopenia) with normal bone marrow and the absence of other causes of thrombocytopenia.diopathic thrombocytopenic purpura (ITP), also known as primary immune thrombocytopenia, primary immune thrombocytopenic purpura or autoimmune thrombocytopenic purpura, is defined as isolated low platelet count (thrombocytopenia) with normal bone marrow and the absence of other causes of thrombocytopenia.