Thrombocytopenia is defined as a low circulating platelet count (<150,000 per microlitre). Platelet life span is normally approximately 5 days, with continual renewal. True thrombocytopenia results from either a reduced production of platelets in the bone marrow, increased clearance, or sequestering of platelets in the spleen.
The causes of thrombocytopenia are diverse, making epidemiological generalisations difficult. However, immune thrombocytopenia (ITP) is typically an illness of young women.   The apparent appearance of ITP in other patient groups should prompt a high index of suspicion for an alternative diagnosis: for example, non-Hodgkin's lymphoma, drug effect, or myelodysplastic syndrome.
A complete history, physical examination, review of the FBC, and peripheral smear are necessary to narrow down the differential diagnosis of thrombocytopenia. It is important to exclude artifact or pseudothrombocytopenia.
Weill Cornell Medicine
Pathology and Laboratory Medicine
Division of Immunopathology
JTG declares that she has no competing interests.
Dr Julia T. Geyer would like to gratefully acknowledge Dr Alan E. Lichtin, a previous contributor to this monograph. AEL is on an advisory board and has received research support from Amgen.
Clinical Professor of Medicine
University of Texas Health Science Center
Cancer Care Network of South Texas
RML declares that he has no competing interests.
Clinical Assistant Professor of Medicine
University of Connecticut School of Medicine
JSW has received research support from Amgen, Ligand Pharmaceuticals, and MGI Pharma for clinical studies in immune thrombocytopenic purpura.
Professor of Medicine
Case Western Reserve University School of Medicine
KRM declares that he has no competing interests.
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