Overview of Adult Immune Thrombocytopenia

Abstract

There have been somewhat divergent criteria for making the diagnosis of immune thrombocytopenia
(ITP). However, an attempt to standardize the definition of ITP has been made with the
publication by the International Work Group (IWG) on ITP in 2009.1 Despite this attempt to
standardize those descriptions, many physicians do not conform to these definitions, so confusion
in terminology persists. After publication of the IWG guideline, most use immune thrombocytopenia.
Its various other names have included “idiopathic thrombocytopenic purpura”,
“autoimmune thrombocytopenic purpura”, “immune thrombocytopenic purpura”, “autoimmune
thrombocytopenia”, and “primary thrombocytopenic purpura”. Despite what it is called,
it represents an acquired autoimmune process and is no longer considered idiopathic. Autoantibodies
are not only directed against the patient’s own platelets, leading to their destruction by
the spleen, but there may be humoral autoimmune processes involving the platelet progenitor,
the megakaryocyte, resulting in decreased platelet production. In the 1st issue of Blood, published
in 1946, Damasheck and Miller2 reviewed the megakaryocyte counts and bone marrow
morphology of patients with idiopathic thrombocytopenia and demonstrated most had an increase
in the megakaryocyte count, however few platelets were produced. Multiple Fc receptor
(FcRs) pathways are involved in the humoral immune response associated with ITP. It has also
been discovered that cellular autoimmune disruption contributes to thrombocytopenia in ITP.
The sole abnormality on the hemogram is a platelet count <100,000/μL (formerly <150,000/
μL) in the setting of an otherwise normal peripheral blood with normal red cell and white cell
morphology in a healthy person. The peripheral blood platelets can range from normal to large
in size. The decrease in the platelet cut-off from 150,000 to 100,000/μL by the IWG was based
on a long-term outcome study of healthy individuals, where it was found that those with platelet
counts between 100,000 to 150,000/μL had a 10 year probability of developing ITP (a persistent
platelet count below 100,000/μL) of only 6.9%.1 Additionally, it was noted that platelet
counts of 100,000 to 150,000/μL in non-Western populations are found with some frequency in
healthy people. Immune thrombocytopenia remains the most common cause of isolated thrombocytopenia
in clinical practice.