Implementation of Flow Cytometric Monocyte Repartition Assay for Assessment of Chronic Myelomonocytic Leukaemia (CMML) in Bone Marrow

Background: CMML is a myelodysplastic/myeloproliferative overlap syndrome associated with persistent monocytosis. Previous studies have shown a characteristic phenotype of peripheral blood (PB) monocytes in CMML that may have diagnostic utility. There is expansion of the classical monocyte (Mo1) population (CD14+CD16-) with concomitant reduction in the intermediate (Mo2; CD14+16+) and non-classical (Mo3; CD14-16+) populations. It is unclear whether an increased Mo1 population or a reduced Mo2/Mo3 population is a better discriminator in detecting CMML by flow cytometry. Diagnostic thresholds for these populations have most often been established by comparison with normal donors using PB samples. The objective of this study was to determine the optimal cut-off limits for flow cytometric monocyte repartition (FMR) in bone marrow (BM) samples and to assess the diagnostic accuracy of this method in patients undergoing BM evaluation for other causes of monocytosis using a novel gating strategy. Patients/Methods: 71 BM aspirates were analysed comprising 35 confirmed CMML (21 proliferative CMML , 14 dysplastic CMML), 13 other myeloid neoplasm (MN) disease controls and 23 normal bone marrow aspirates. The MN cohort was identified through local samples with a known peripheral blood monocytosis of ≥1.0x109 and peripheral blast count of Stored flow cytometric data were reanalysed to assess monocyte subsets. After initial removal of doublets and non-cellular events, sequential gating selected a CD45+CD33bright population with low side scatter to select monocytes and exclude granulocytes. The lymphocyte gate as determined on the CD45 vs CD33 plot was used to establish a threshold for CD16 positivity. Immature double negative monocytes (CD14-CD16-) were excluded as in published gating strategies. Results: Linear regression analysis of the percentage of both the Mo1 and Mo2 populations demonstrated a significant difference between the CMML, MN, and normal cohorts (global P 88.5% (Figure 1B) gave a sensitivity of 77.1% (95% CI: 60.98%, 87.93%) and specificity of 88.9% (95% CI: 74.69%, 95.59%) (Figure 1A). A Mo2% cut-off value of Conclusion: Our study confirms that analysis of monocyte subsets on BM samples provides good discrimination of CMML cases when compared to normal controls. There was significant overlap with other MN samples. FMR may be a useful adjunct to BM morphology in cases with subtle dysplasia. Further study is required to understand the overlap in monocyte phenotype between CMML and other myeloid neoplasms. Download : Download high-res image (254KB) Download : Download full-size image Disclosures Allen: Amgen: Other: Travel Support; Roche: Other: Travel Support; Novartis: Other: Travel Support. Hahn: Roche: Honoraria; Astra Zeneca: Honoraria.