Immune-mediated thrombocytopenia

Immune-mediated thrombocytopenia is the term used to describe the group of thrombocytopenic disorders in which platelets are destroyed by immune mechanisms. The thrombocytopenia develops when the rate of platelet destruction is greater than the ability of the megakaryocytes in the bone marrow to compensate with increased platelet production. Different investigators have attempted to quantitate this rate of platelet destruction; however, the estimates are relatively imprecise. Many investigators feel that the marrow can compensate at least fivefold without a detectable fall in the platelet count. The platelet destruction in immune-mediated thrombocytopenia can be caused by autoantibodies, alloantibodies, and immune complexes. These are illustrated schematically in Fig. 36.1. Most episodes of immune-mediated platelet destruction are caused by the binding of IgG antibodies to platelet-specific membrane components.

General approach to a patient with suspected immune-mediated thrombocytopenia

When a patient presents with suspected immune-mediated thrombocytopenia, the physician must simultaneously confirm that the patient is thrombocytopenic, begin to determine the general mechanism responsible for the thrombocytopenia, and determine what, if any, treatment is required. Although these various aspects of thrombocytopenia are covered elsewhere in this book, it is appropriate to briefly address each question.

Is the patient thrombocytopenic?

Today, most platelet counts are performed using automated particle counters. With cutbacks in budgets for health care, it has become far less common for every blood film of suspected thrombocytopenic patients to be examined, and even less common for manual (phase-contrast) platelet counts to be performed.