Myeloblasts in normal bone marrows expressing leukaemia-associated immunophenotypes

Measurable residual disease (MRD) status of patients undergoing treatment for acute myeloid leukaemia (AML) is important for prognosis and guides treatment. Multicolour flow cytometry (MCF) is a sensitive MRD method. The current approach relies on identification of blasts expressing leukaemia-associated immunophenotypes (LAIP) or by blasts expressing aberrant differentiation/maturation profiles compared to that seen in normal haematopoietic precursor cells at follow-up, i.e., different from normal (DFN). However, expression of LAIP on normal myeloblasts affects the specificity of the result, and the understanding of what is normal is important. Limited published data are currently available. We report findings from 14 normal adult bone marrows. MCF was performed on the residual normal marrow specimens from 14 adults. Expression of CD15, CD11b, CD7, CD4, and CD56 on CD34+ myeloblasts was assessed. Analysis of samples was performed using 4-colour flow cytometry which was the methodology used when this work was done, and is still being used in many clinical flow laboratories worldwide. LAIP is defined by lineage infidelity or asynchronous expression of differentiation markers. The cases of normal myeloblasts with LAIP involving the markers used and above the cut-off levels for MRD detection (0.01%) varies between 43% and 100%, limiting the specificity of the results for MRD. Even if the threshold is raised to 0.1%, there will still be false positive cases using aberrant CD15 or CD7. Our work provided useful information for AML MRD determination in our laboratory. A collaborative database of LAIP on normal myeloblasts using standardised analysis should be useful to determine the optimal diagnostic cut-off for AML MRD using LAIP.