Is refractory anaemia with ring sideroblasts and thrombocytosis (RARS-T) a necessary or useful diagnostic category?

Both the 2001 World Health Organisation (WHO) classification of haematopoietic neoplasms and the 2008 WHO classification revision include a distinctive diagnostic category, refractory anaemia with ring sideroblasts and thrombocytosis (RARS-T), to describe those rare patients who have both >or=15% ring sideroblasts and a sustained elevated platelet count. Recently, it has become clear that patients meeting WHO criteria for RARS-T have clonal JAK2(V617F) and MPL(W515) mutations at a similar rate to essential thrombocythaemia (ET). Given that the provisional classification of RARS-T as a myelodysplastic syndrome/myeloproliferative neoplasm (MDS/MPN) overlap syndrome, rather than as a form of MPN (i.e., ET), rests principally upon the presence of ring sideroblasts, which are a non-specific morphological finding, these new molecular results prompt reconsideration of the necessity for a distinctive RARS-T category. Here we review the historical developments that led up the definition of RARS-T as a disease entity, and we discuss conceptual understanding of RARS-T and arguments against continued use of RARS-T as a separate diagnostic category.