Morphologic Confounders and CD19 Negativity in a Case of Hairy Cell Leukemia

OBJECTIVES We report a case of hairy cell leukemia (HCL) initially misdiagnosed as plasma cell dyscrasia due to various clinical, morphological and immunophenotypic confounders. METHODS AND RESULTS In a patient diagnosed of marrow plasmacytosis and serum monoclonal protein elsewhere and referred to our hospital, morphological evaluation of bone marrow aspirate smears and trephine biopsy, immunophenotyping, and molecular testing (BRAFV600E mutation) were done. Clinically, the patient was asymptomatic; bone marrow revealed plasmacytosis, mastocytosis, and lymphocytosis with a few "hairy" cells. Immunophenotyping showed features of HCL with aberrant CD10 expression and a large subclone of CD19neg cells. A diagnosis of HCL with reactive plasmacytosis and mast cell hyperplasia was made and confirmed by immunophenotyping and molecular studies. CONCLUSION Hematopathologists must be aware of various confounding factors and should judiciously use flow cytometric and molecular studies for attaining a proper diagnosis of HCL. We also report a very rare immunophenotypic aberrancy (CD 19 negativity) in HCL.