Mimics and artefacts of measurable residual disease in a highly sensitive multicolour flow cytometry assay for B-lymphoblastic leukaemia/lymphoma: critical consideration for analysis of measurable residual disease.

High-sensitivity multicolour flow cytometry (MFC)-based B-lymphoblastic leukaemia (B-ALL) measurable residual disease (BMRD) assay is increasingly being used in clinical practice. Herein, we describe six consistently present low-level populations immunophenotypically mimicking abnormal B-ALL blasts in 441 BMRD samples from 301 children. These included CD19 ⁺ CD123 ⁺ plasmacytoid dendritic cells differentiating from lymphoid precursors, CD10 ⁺ transitional B cells with CD10 ⁺ /CD38dim-to-negative/CD20bright/CD45bright phenotype, CD19 ⁺ natural killer (NK) cells, CD73bright/CD10 ⁺ mesenchymal stromal/stem cells, CD73bright/CD34 ⁺ endothelial cells, and a CD34 ⁺ CD38dim-to-negative/CD10 ^- /CD20bright/CD45bright subset of mature B cells. We provide the proportions, comprehensive immunophenotype, and practical clues for proper identification of these low-level populations. Knowledge regarding the presence and immunophenotype of these mimics is essential for accurate interpretation in high-sensitivity MFC-BMRD analysis.
(© 2021 British Society for Haematology and John Wiley & Sons Ltd.)