Diagnosis and Reclassification of Myelodysplastic Syndromes and Acute Myeloid Leukemia: Comparison Between the 2016 WHO 4th and 2022 WHO 5th/ICC Classifications

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Background: In 2022, the World Health Organization (WHO) 5th and International Consensus Classification (ICC) classification of hematologic malignancies including acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS) was introduced.This study contrasted the diagnosis and reclassification of MDS and AML between the 2016 WHO 4th and WHO 5th/ICC classifications.Methods: From April 2019 to June 2023, 246 MDS and AML patients diagnosed by the WHO 4th classification at the Ulsan University Hospital were included in this study.These patients were reclassified by the WHO 5th and ICC classification, and the differences were retrospectively compared and analyzed.Results: Among 96 MDS patients diagnosed by the WHO 4th classification, 5 (5.2%) were reclassified to AML using the WHO 5th or ICC due to concomitant cytogenetic and molecular abnormalities.Only 2 patients initially diagnosed with MDS with excess blasts-2 (MDS-EB-2) were later diagnosed with AML by ICC classification.When the WHO 5th and ICC classifications were applied, 28.6% and 17.0%, respectively, of MDS patients diagnosed by the WHO 4th classification changed MDS subtypes, mostly due to the presence of molecular abnormalities such as SF3B1 and biallelic TP53 mutation at diagnosis.The MDS subtypes of certain patients also changed when the WHO 5th classification were applied due to the addition of the MDS-hypoplastic and MDS-fibrosis categories.Conclusions: When applying the WHO 5th and ICC classifications for the diagnosis of MDS and AML, morphologic findings of bone marrow biopsy and reduction of turnaround time in cytogenetic and molecular genetic tests would be crucial in improving diagnostic accuracy.