Myelodysplastic syndromes (MDS) are a heterogeneous group of clonal hematopoietic disorders that are divided into diagnostic categories primarily based on the percentage of blast cells, the presence of ringed sideroblasts, and the cell lineage affected by dysplasia.
In MDS, the presence of clusters of blasts in nonparatrabecular or perivascular location (also termed abnormal localization of immature precursors) are associated with a poor prognosis. CD34 can be used to highlight the presence of abnormal localization of immature precursors (Fig. 9) (12,16,17). This finding, as well as an increase in the percentage of CD34-positive cells, is associated with a higher frequency of leukemic transformation and short survival in MDS cases. The occurrence of large sheets of CD34-positive blasts confirms transformation AML. Similar findings may be associated with transformation of CML to acute leukemia (18). Immunostaining for CD34 is especially important in two subsets of patients with MDS: MDS with fibrosis and MDS with hypocellular marrow (MDS-h). MDS-f is characterized by the presence of significant degrees of marrow fibrosis, which preclude aspiration for either flow cytometric or cytogenetic analysis. In these cases, the presence of increased CD34 expression in the marrow is often helpful in the diagnosis (most cases of MDS-f have an excess of blasts). In MDS-h, CD34 can be used to distinguish hypoplastic MDS from acquired aplastic anemia (Fig. 10). The former disorder is characterized by higher CD34 counts as compared with aplastic anemia (19).
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