As indicated by the title of this chapter, some conditions may display features evocative of hematological malignancies and have to be recognized as not being tumoral. Here, these situations have been grouped as increased leucocyte types (leucocytosis) or as decreased cell counts (cytopenias), segregated in disease types. A third part considers abnormal immunophenotypes of lymphocytes and myeloid cells. Finally, the recurrent question of haemodilution of bone marrow aspirates, which decreases the otherwise helpful ability of flow cytometry to count large numbers of cells and thus perform accurate differentials, is discussed.

Myelodysplastic syndromes (MDS) comprise a heterogeneous group of myeloid neoplasms defined by peripheral cytopaenia, morphologic dysplasia in one or more haematopoietic lineages, and genetic instability with risk of transformation to acute myeloid leukaemia (AML). A diagnosis of MDS requires the presence of cytopaenia, plus one or more of the following diagnostic features: morphologic evidence of dysplasia in >10% of cells involving at least one haematopoietic lineage, increased blasts, and/or an MDS-defining cytogenetic abnormality. Modern diagnosis of MDS was standardized by the French–American–British (FAB) cooperative group in 1982, which included the first standard descriptions of myelodysplasia [1], and was further refined in the World Health Organization (WHO) classification of 2001, which more specifically quantified dysplasia (Table 8.1) [2].