1 Division of Hematology, Department of Medicine, Nippon Medical School, Tokyo, Japan 2 Department of Hematology Oncology, University of Pavia Medical School and Fondazione IRCCS Policlinico San Matteo, Pavia, Italy 3 Division of Hematology, Department of Internal Medicine, Chiba Hokusoh Hospital, Nippon Medical School, Chiba, Japan 4 Department of Hematology, Saitama I nternational Medical Center, Saitama Medical University, Saitama, Japan 5 Department of Industrial Science and Technology, Tokyo University of Science, Chiba, Japan 6 First Department of Internal Medicine, School of Medicine, University of Occupational and Environmental Health, Fukuoka, Japan Correspondence: Kiyoyuki Ogata, Division of Hematology, Nippon Medical School, 1-1-5 Sendagi, Bunkyoku, Tokyo 113-8603, Japan. E-mail: ogata{at}nms.ac.jp Background: The diagnosis of myelodysplastic syndromes is not always straightforward when patients lack specific diagnostic markers, such as blast excess, karyotype abnormality, and ringed sideroblasts. Design and Methods: We designed a flow cytometry protocol applicable in many laboratories and verified its diagnostic utility in patients without those diagnostic markers. The cardinal parameters, analyzable from one cell aliquot, were myeloblasts (%), B-cell progenitors (%), myeloblast CD45 expression, and channel number of side scatter where the maximum number of granulocytes occurs. The adjunctive parameters were CD11b, CD15, and CD56 expression (%) on myeloblasts. Marrow samples from 106 control patients with cytopenia and 134 low-grade myelodysplastic syndromes patients, including 81 lacking both ringed sideroblasts and cytogenetic aberrations, were prospectively analyzed in Japan and Italy. Results: Data outside the predetermined reference range in 2 or more parameters (multiple abnormalities) were common in myelodysplastic syndromes patients. In those lacking ringed sideroblasts and cytogenetic aberrations, multiple abnormalities were observed in 8/26 Japanese (30.8%) and 37/55 Italians (67.3%) when the cardinal parameters alone were considered, and in 17/26 Japanese (65.4%) and 42/47 Italians (89.4%) when all parameters were taken into account. Multiple abnormalities were rare in controls. When data from all parameters were used, the diagnostic sensitivities were 65% and 89%, specificities were 98% and 90%, and likelihood ratios were 28.1 and 8.5 for the Japanese and Italian cohorts, respectively. Conclusions: This protocol can be used in the diagnostic work-up of low-grade myelodysplastic syndromes patients who lack specific diagnostic markers, although further improvement in diagnostic power is desirable. Key words: myelodysplastic syndromes, flow cytometry, diagnosis. Related Article Flow cytometry immunophenotyping for diagnosis of myelodysplastic syndrome Mario Cazzola Haematologica 2009 94: 1041-1043. Full Text PDF