Immunophenotypic abnormality quantification refines multiparameter flow cytometry‐based measurable residual disease testing in adults allografted for acute myeloid leukemia in morphologic remission

Flow cytometry-based measurable residual disease (MRD) testing is routinely used in acute myeloid leukemia (AML), but methodologies need refinement to optimize assay characteristics. Here, we examined 1215 adults with AML or myelodysplastic syndrome/AML allografted in morphologic remission to study how the type(s) of leukemic blasts and degree/number of immunophenotypic abnormalities could improve MRD testing. Among 233 patients with pre-hematopoietic cell transplantation (pre-HCT) MRD, 80 (34%) had non-stem cell-like (NSC-like) leukemic blasts, 109 (47%) had stem cell-like (SC-like) leukemic blasts, and 44 (19%) had NSC- and SC-like leukemic blast cell populations. Across all MRD^pos patients, a higher degree/number of immunophenotypic abnormalities was associated with increased relapse risk and worse relapse-free survival (RFS) and overall survival (OS). Maximally selected rank statistics estimated a total MRD immunophenotype score cut point of ≤4.5 (<i>n</i> = 63 [27% of MRD^pos patients]) vs. >4.5 (<i>n</i> = 170 [73% of MRD^pos patients]) as optimal for RFS discrimination. After multivariable adjustment, a high score was associated with a significantly increased relapse risk (hazard ratio [HR] = 4.99 [95% confidence interval: 3.92-6.36]; P < 0.001), shorter RFS (HR = 3.88 [3.15-4.78]; P < 0.001), shorter OS (HR = 2.99 [2.42-3.70]; P < 0.001), and higher risk of NRM (HR = 1.78 [1.07-2.81]; P = 0.014) relative to MRD^neg patients. In contrast, there was no significant relapse risk or RFS difference between patients with low total MRD immunophenotype score and those without MRD. While requiring validation, our data suggest considering the type and degree/number of immunophenotypic abnormalities may refine MRD testing and identify a significant subset of MRD^pos patients with outcomes like MRD^neg patients.