With the exception of chronic myelogenous leukemia (CML), which is characterized by the constitutively active chimeric bcr-abl tyrosine kinase, the diagnosis of the myeloproliferative disorders (MPD), polycythemia vera (PV), essential thrombocytosis (ET), and idiopathic myelofibrosis (IMF), is unaided by specific biologic markers and is complicated by phenotypic mimicry. Diagnosis is largely clinical with assistance from conventional laboratory techniques. Although the sine qua non of PV is erythrocytosis, survey data indicate that approximately 22% of American Society of Hematology members diagnose PV without the benefit of red blood cell mass and plasma volume measurements. These tests are essential in any suspected case of PV because erythrocytosis can be masked by an elevated plasma volume in this disorder. Recognition of defective platelet thrombopoietin receptor (Mpl) expression and overexpression of the PRV-1 gene in PV neutrophils has led to studies demonstrating the potential use of these molecular abnormalities as diagnostic markers for PV and for risk-stratifying patients with ET for disease conversion to PV or IMF. Enumeration of peripheral blood CD34+ cells may prove useful in the diagnosis of IMF if current data regarding the disease-related specificity of this measurement are validated prospectively in larger numbers of patients.
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