A 30-year-old woman presented with fever and fatigue. She had pallor, gum hyperplasia, and hepatosplenomegaly along with anemia (hemoglobin 7.7 g/dL), thrombocytopenia (63×10⁹/L), and leukocytosis (20.7×10⁹/L). Her marrow aspirate had 76% blasts, many showing single, long, slender Auer rods. Dysgranulopoiesis was noted (A). There was a marked increase in hemophagocytic histiocytes, some of them showing the presence of multiple ingested intact Auer rods (B, C). The histiocytes imparted a “starry sky appearance” to the trephine biopsy (D). Immunophenotyping using flow cytometry showed weak CD45-positive blasts that were positive for CD13, CD33, CD117, CD38, HLA-DR, and anti-MPO antibody. They were negative for CD41, CD61 (megakaryocytic), CD14, CD64 (monocytic), T and B cell antigens. Induction chemotherapy was initiated.

Multiple ingested intact Auer rods in histiocytes may be seen in promyelocytic leukemias during therapy. Auer rods are composed of the myeloperoxidase enzyme in a crystalline form, which may resist digestion by the enzymes in the histiocytes and thus persist. Extensive phagocytic activity, as evidenced by multiple Auer rods within the macrophages in non-promyelocytic leukemias as seen in this case, is an unusual occurrence.