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Highlights • Microscopic examinations were performed separately by two morphologists. • Interobserver concordance of the percentage of blasts (<5% or ≥5%, ≤2% or >2–<5%) was found. • Interobserver concordance of dysplastic lineages was found. • Interobserver concordance of the diagnosis (MDS or non-MDS) was found. Abstract The diagnosis of myelodysplastic syndromes (MDS) is based on morphology and cytogenetics. However, limited information is currently available on the interobserver concordance of the assessment of dysplastic lineages (<10% or ≥10% in bone marrow (BM)). The revised International Prognostic Scoring System (IPSS-R) described a new threshold (2%) for BM blasts. However, the interobserver concordance of the categories (0–≤2% and >2–<5%) has limited data. The purpose of the present study was to investigate the assessment of dysplastic lineages and IPSS-R reproducibility. Our study was divided into two Steps. In each Step, the microscopic examinations were performed separately by two morphologists. Regarding the category of BM blasts ≤2% and >2–<5%, interobserver agreement was more than 'moderate' in all pairs (kappa test: 0.43–0.90). Regarding dysgranulopoiesis (dysG) and dyserythropoiesis (dysE) in BM, interobserver agreement was more than 'moderate' in all pairs (kappa test, dysG: 0.45–0.96, dysE: 0.45–0.81). Regarding the category of dysmegakaryopoiesis (dysMgk) in BM, interobserver agreement was more than moderate in 4 out of 5 pairs (kappa test: 0.58–1.00), and was fair for one pair (kappa test: 0.37). We consider that high interobserver concordance may be possible for the BM blast cell count (≤2% or >2–<5%) and dysplasia (<10% or ≥10%) of each lineage. [ABSTRACT FROM AUTHOR]