Challenges and Advances in the Detection of Leukemic Blasts in Cerebrospinal Fluid in Pediatric Acute Lymphoblastic Leukemia

Central nervous system (CNS) evaluation for leukemic involvement is essential both at initial diagnosis and throughout relapse surveillance in childhood acute lymphoblastic leukemia (ALL). Accurate CNS risk classification is a cornerstone of individualized chemotherapy and has significantly advanced treatment strategies. However, detecting leukemic cells in the cerebrospinal fluid (CSF) is challenging, particularly when only a small number of cells are present. While cytomorphology remains a standard diagnostic method, it is limited by low sensitivity and interobserver variability, especially in low-cellularity or equivocal samples. Flow cytometry offers superior sensitivity and specificity and is increasingly recommended to confirm or clarify ambiguous findings. Current guidelines support the use of both cytomorphologic review and flow cytometry to maximize diagnostic accuracy. Evidence consistently demonstrates that any detectable CSF blasts-even in the setting of low WBC counts-are associated with increased risk of CNS relapse and poorer outcomes, underscoring the importance of risk-adapted CNS-directed therapy. Although the prognostic significance of isolated flow-only positivity remains under study, emerging data suggest that timely therapeutic intensification may mitigate adverse outcomes. Additional modalities, including advanced flow cytometry and molecular assays, may further refine CSF assessment in the future. This review summarizes current diagnostic approaches and highlights the need for standardized protocols for CSF evaluation in pediatric ALL.