Auer rods

Auer rods are crystalline aggregates of myeloperoxidase and other granule contents (lysosomal enzymes) that form within the cytoplasm of myeloid blasts. They stain positive with myeloperoxidase (MPO) and Sudan black B stains, which helps distinguish myeloid from lymphoid blasts on peripheral smear or bone marrow biopsy. Numerous Auer rods bundled together, often called 'faggot cells,' are highly characteristic of acute promyelocytic leukemia (APL, AML M3), which is associated with the t(15;17) translocation involving the PML-RARA fusion gene. This subtype carries a high risk of disseminated intravascular coagulation (DIC) due to release of procoagulant granule contents, making prompt recognition critical—treatment with all-trans retinoic acid (ATRA) induces differentiation of the promyelocytes and can be life-saving, often combined with arsenic trioxide. Clinically, identifying Auer rods on a blood smear or bone marrow aspirate supports a diagnosis of AML over ALL, guiding both diagnostic workup (flow cytometry, cytogenetics) and urgent therapeutic decisions, especially avoiding heparin-based prophylaxis and initiating ATRA immediately if APL is suspected due to the bleeding risk.