Chronic myeloid disorders (CMD) are collectively characterized by monoclonal myeloproliferation that involves multiple lineages, retains a variables degree of cellular maturation, and has the potential to undergo clonal evolution. However, monoclonal hematopoiesis is neither essential nor specific to CMD. Morphologic and cytogenetic characteristics allow a working classification of these disorders that is clinically useful. There are four major divisions: chronic myeloid leukemia (CML), which is easily identified by the presence of the Philadelphia chromosome (or its molecular equivalent); the myelodysplastic syndromes (MDS), which are characterized by trilineage dysplasia; chronic myeloproliferative diseases (CMPD), which include essential thrombocythemia, polycythemia vera, and agnogenic myeloid metaplasia (AMM); and atypical CMD, which includes chronic neutrophilic leukemia, chronic eosinophilic leukemia, mast cell disease, and myeloid processes that display overlapping features of MDS and CMPD (hybrid CMD). In CMPD, a diagnosis of polycythemia vera requires evidence of an erythropoietin-independent increase in red blood cell mass; the diagnosis of both AMM and essential thrombocythemia requires the exclusion of reactive causes of bone marrow fibrosis and thrombocytosis, respectively. In addition, the Philadelphia chromosome, increased red blood cell mass, and dyserythropoiesis should also be absent.
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