Atypical presentation of hairy cell leukemia: Significance of CD200 on flow cytometry.

Hairy cell leukemia (HCL) is a rare, low-grade mature B-cell neoplasm with a characteristic clinical, morphological, immunophenotypic, and more recently described molecular (BRAF p.V600E mutation) profile. It typically affects middle-aged to elderly male who present with pancytopenia and massive splenomegaly. Lymphadenopathy is usually not seen. Atypical presentations such as absence of splenomegaly and presence of lymphadenopathy and leukocytosis, a hypoplastic marrow masquerading as aplastic anemia, pose a diagnostic challenge to both clinician and pathologist. A diligent morphological examination to look for the presence of hairy cells along with flow cytometric immunophenotyping showing consistent bright expression of CD200, in addition to well-described characteristic immunophenotype, helps in correctly diagnosing the case. This can be further confirmed by the consistent presence of V600E point mutation in BRAF gene. The correct identification of HCL in these unusual clinical presentations is of utmost importance owing to a different treatment approach in these cases. We present here four such cases with atypical presentation.