World Health Organization Classification of Myelodysplastic Syndromes

Myelodysplastic syndromes (MDS) are characterized by an ineffective hematopoiesis and functional abnormalities of hematopoietic lineages. Patients with MDS present with cytopenia(s) associated with morphological dysplasia and /or increase in number of blasts, and can progress to acute myeloid leukemia. The pathogenesis of MDS is exceedingly complex and involves the hematopoietic stem cells, bone marrow microenvironment and an interaction between these two compartments. The laboratory diagnostic strategy in MDS has evolved significantly over the years from a purely morphological one based almost exclusively on peripheral blood smear and bone marrow aspirate cytology to the integrated multiparametric approach used in the 2001 and 2008 WHO classification schemes. An evolution has also occurred in the prognostic assessment and the evaluation of treatment response to include novel disease related factors as well as patient specific characteristics such as non-hematologic comorbidities. This progress in clinically relevant subclassification of MDS and inclusion of specific variables related to treatment response are particularly important considering the increasing treatment options available for MDS. This review is largely focused on the diagnostic approach to MDS including discussion of the significance of cytogenetic/genetic and immunophenotypic features. We present the overview of the minimal diagnostic criteria for diagnosing MDS and a detailed description the current World Health Organization (WHO) classification scheme.