Acute myeloid leukemia pretreated with filgrastim mimicking acute promyelocytic leukemia.

A 66-year-old man with a history of hepatitis C virus and liver transplantation presented to the emergency department for shortness of breath. He had received weekly filgrastim for chronic neutropenia, and immediately before admission he had received 3 days of filgrastim and 1 day of pegfilgrastim. A hemogram showed a white blood cell count of 4.54 K/mL with 23% blastic cells. Emergent bone marrow specimen revealed 50% blasts and 43% promyelocytes with Auer rods (panel A). Flow cytometry demonstrated a predominant population expressing myeloid antigens, CD38, CD117, and dim CD7, but not HLA-DR or CD34. These features suggested acute promyelocytic leukemia (APL) and treatment with all-trans-retinoic acid (ATRA) was initiated. Subsequent fluorescence in situ hybridization testing for promyelocytic leukemia (PML)/retinoic acid receptor alpha (RARa) with dual-fusion and RARa break-apart probes was negative (panel B). Molecular testing for PML/ RARa messenger RNA by reverse-transcriptase polymerase chain reaction was also negative, but FLT3-ITD was positive. Despite morphologic and immunophenotypic evidence for APL, confirmatory cytogenetic and molecular tests were negative; therefore, acute myeloid leukemia–not otherwise specified was the final diagnosis.