Prognostic significance of end‐of‐induction measurable residual disease below 0.01% in pediatric B‐lineage acute lymphoblastic leukemia

End-of-induction measurable residual disease (EOI-MRD) predicts poor outcomes in B-lineage acute lymphoblastic leukemia (B-ALL). While MRD positivity is conventionally defined at ≥0.01%, data regarding the prognostic relevance of flow cytometry derived EOI-MRD of <0.01% are emerging. Consecutive pediatric B-ALL patients diagnosed between January 2018 and December 2022 were analyzed. Overall survival (OS), relapse-free survival (RFS), and cumulative risk for relapse (CRR) were compared according to undetectable, <0.01%, and ≥0.01% EOI-MRD status. Of 223 patients, 81 (36%) had detectable EOI-MRD, including 18 (22.4%) with MRD <0.01%. At 60 months, any detectable EOI-MRD was associated with higher CRR (44% vs. 22%, p < 0.001) and inferior OS (57% vs. 83%, p < 0.001) and RFS (56% vs. 78%, p < 0.001) compared with undetectable MRD. Outcomes were similar between MRD <0.01% and ≥0.01% groups (CRR 44% vs. 44%, OS 66% vs. 59%, RFS 56% vs. 56%; all p > 0.05). Compared with undetectable MRD, MRD <0.01% was associated with adverse OS (83% vs. 66%, p = 0.016) and RFS (78% vs. 56%, p = 0.029). On multivariate analysis, any level of detectable EOI-MRD independently predicted adverse outcomes (HR: 2.22, 95% CI: 1.30-3.8; p = 0.004 for OS and HR: 2.38, 95% CI: 1.42-3.98; p = 0.001 for RFS) [CI: confidence interval and HR: hazard ratio]. EOI-MRD <0.01% is associated with inferior outcomes. Whether treatment intensification based on EOI-MRD <0.01% improves outcomes requires prospective evaluation.