Myelodysplastic syndromes versus acute myeloid leukaemia: biology or blasts—what truly defines the disease and does it matter?

The distinction between myelodysplastic syndromes and acute myeloid leukaemia remains a subject of debate, with direct implications for clinical decisions, trial eligibility, and allogeneic hematopoietic cell transplantation allocation. Although the historical 20% blast threshold in bone marrow is commonly used to separate myelodysplastic syndromes from acute myeloid leukaemia, emerging evidence shows that morphological, cytogenetic, and molecular features provide a more accurate framework for diagnosis and risk stratification. This Viewpoint discusses how advances in genomics and molecular biology have reshaped the classification of these disorders, highlighting molecular signatures and germline alterations as key tools to guide therapeutic decisions, transplant eligibility, and measurable residual disease assessment. We emphasise that myelodysplastic syndromes represent a biologically distinct entity to acute myeloid leukaemia, reinforcing the need to tailor treatment strategies according to disease biology to optimise patient outcomes.