[Classification and diagnosis of essential thrombocythemia].

Although the fourth edition of the World Health Organization (WHO) classification of the diagnostic criteria for myeloproliferative neoplasms is widely accepted, the diagnosis of essential thrombocythemia has been debated. One of the controversies focuses on the provisional entity known as refractory anemia, which exhibits ringed sideroblasts associated with marked thrombocytosis (RARS-T). To date, it has not been made clear whether RARS-T is a form of ET or a separate and unique disease entity. Recent findings, which may shed light on this debate, have demonstrated that more than 60% of RARS-T patients exhibit somatic mutations of SF3B1, a gene encoding a core component of the RNA-splicing system. Another debate is focused on the role of bone marrow histopathology in the diagnosis of essential thrombocythemia. Bone marrow histopathology has been considered to be a major diagnostic criterion for essential thrombocythemia since the previous version of the WHO classification criteria was published. The histological features of bone marrow are important to distinguish essential thrombocythemia from early, prefibrotic stages of primary myelofibrosis, which frequently exhibit elevated platelet levels. The specific histological bone marrow patterns of the diseases were outlined in the fourth version of the WHO classification; however, several investigators have claimed that the criteria exhibit poor interobserver reproducibility and are not sufficiently robust to discriminate essential thrombocythemia and early-stage primary myelofibrosis. This review highlights the recent debates on the role of bone marrow histopathology in essential thrombocythemia.