Monocytopenia as a diagnostic clue to pediatric B-lymphoblastic leukemia with rare circulating blasts.

B-lymphoblastic leukemia/lymphoma (B-LL) is the most common childhood cancer. Circulating blasts in the peripheral blood may be rare (≤1%) and missed, even when flow cytometric immunophenotyping is performed, leading to a false-negative report. The records from all patients with a new diagnosis of B-LL between January 2009 and December 2011 at our institution were reviewed. Of 130 cases with peripheral blood flow cytometry, 15 had a blast count of ≤1%, with 14 having electronic files for gating monocytes. The percentage of monocytes by flow cytometry and absolute monocyte counts (AMCs) were compared with peripheral blood samples that were negative by flow cytometry, sent due to cytopenia of at least 1 lineage (n  =  39). The monocytes from the patients with leukemia averaged 0.8% and were statistically fewer than the negative controls, which averaged 7.1% (P < 0.001). Eleven of the 14 (79%) patients with leukemia had monocytes <1%, compared to only 3 (8%) of the negative controls. The AMCs were also significantly lower (P < 0.001), with 93% of the leukemia group having an AMC <100 cells/µL, compared to only 28% of the negative controls. In patients with cytopenias, percentage of monocytes may be an important diagnostic clue in determining the presence of occult leukemia. If flow cytometry is performed, acquisition of more than the standard 10,000 events is necessary to adequately assess for leukemia. If monocytes are <1% by flow cytometry in the setting of cytopenias, bone marrow examination is recommended, even with negative peripheral blood flow cytometry.