Evaluation of BCR::ABL1 Positivity in Peripheral Blood Neutrophils Enables the Diagnosis of De Novo Blast Phase Chronic Myeloid Leukemia with Additional Structural Abnormalities of the Philadelphia Chromosome.

A 60-year-old man with no history of hematologic disease presented with leukocytosis and thrombocytopenia. A bone marrow (BM) examination revealed 85.0% blasts with a B/myeloid immunophenotype and a complex Philadelphia (Ph) chromosome-positive karyotype: 46,XY,t(9;22)(q34;q11.2)[3]/45,XY,der(9)t(9;22),add(19)(p13),-22[17]. Fluorescence in situ hybridization detected single (6%) or duplicated (92%) BCR::ABL1 signals in bone marrow (BM) blasts but only single BCR::ABL1 signals (95%) in peripheral blood (PB) neutrophils. These findings indicate that the duplicated-signal blasts arrested their differentiation, while single-signal blasts retained differentiation potential, supporting a diagnosis of de novo blast phase chronic myeloid leukemia. An evaluation of BCR::ABL1 positivity in PB neutrophils provides valuable diagnostic insights into Philadelphia chromosome-positive leukemias.