Flow cytometry immunophenotypic findings in chronic myelomonocytic leukemia and its utility in monitoring treatment response

Chronic myelomonocytic leukemia (CMML) is a myelodysplastic/myeloproliferative neoplasm, characterized by persistent monocytosis. Due to the lack of unique surface markers expressed by neoplastic monocytes and the frequent CD34‐negative blast immunophenotype, the diagnostic value of flow cytometric immunophenotyping (FCI) in CMML is rarely studied. In this study, by using a multicolor FCI assay, we assessed bone marrow (BM) immunophenotypical alterations in 118 CMML patients and follow‐up BM samples in 35 of these patients. The median BM monocytes as determined by FCI were 14% (1–63%), correlated with morphologic count (P = 0.0004). FCI alterations in monocytes were observed in 96% and granulocytes in 83% of cases. The percentage of CD34⁺ myeloblasts by FCI was low [median 0.6% (0.02–12.6%)], but exhibiting frequent aberrancies [median 6 (2–12)]. CD34⁺ B‐cell precursors were absent in 93% of cases. In 35 patients with follow‐up BM samples assessed, the CD34⁺ myeloblasts showed persistent FCI aberrancies in all 29 patients treated with hypomethylating agents and 3 patients on observation, but became normal in 3 patients following stem cell transplant. In conclusion, CMML exhibit numerous FCI alterations in monocytes, granulocytes, and more profound/frequent in CD34⁺ myeloblasts. These findings provide solid evidence for using FCI as an ancillary test in CMML diagnosis and also, in assessment of treatment responses.