Abnormal Eosinophils With Large, Distinctly Basophilic Granules (Harlequin Cells) on Peripheral Blood Smear: A Clue for Diagnosing Chronic Myeloid Leukemia

Background: fusion, but their diagnostic relevance in CML has not been systematically assessed. Methods: fusion); 11 eosinophilia; and 24 reactive cytosis cases. Harlequin cells were stringently defined as abnormal eosinophils containing both typical eosinophilic granules and large, distinctly basophilic (not purplish-orange) cytoplasmic granules to exclude reactive mimics. Results: fusion and in 20% (3/15) of primary myelofibrosis, but were absent in polycythemia vera, essential thrombocythemia, and chronic myelomonocytic leukemia. Strictly defined Harlequin cells were not found in any reactive condition. Conclusions: confirmatory testing, reduce diagnostic ambiguity, and help avoid unnecessary ancillary studies.