Hairy Cell Leukemia Diagnosed by Flow Cytometry Despite Absence of Hairy Cells in Peripheral Blood: A Case Report

Hairy cell leukemia (HCL) is a rare disease, and detecting hairy cells in peripheral blood is an essential initial step in its diagnosis. Flow cytometry (FCM) is a convenient and effective tool for assessing the immunophenotype, even without distinctive morphological features. We describe a case of HCL; primary myelofibrosis was initially suspected, but FCM findings ultimately led to the diagnosis of HCL. A 47-year-old male patient was admitted to the hospital with progression of pancytopenia, and splenomegaly was noted. The initial pathology report on bone marrow biopsy noted only MF-1 fibrosis. FCM examination showed positivity for CD19, CD20, CD11c, and CD25, and light-chain restriction. Bone marrow biopsy images showed fried egg-like cells, and BRAF V600E mutation was also positive, leading to the diagnosis of HCL. The patient was treated with cladribine and rituximab and achieved hematological recovery. This case highlights the usefulness of FCM for the diagnosis of HCL in the presence of splenomegaly, pancytopenia, and myelofibrosis, even though hairy cells may be absent in peripheral blood.