Auer rod

**What they are:** Auer rods (Auer bodies) are crystalline, needle- or rod-shaped, red-purple (azurophilic) cytoplasmic inclusions seen on Wright-Giemsa smear of myeloid blasts. They are formed by clumping/fusion of primary (azurophilic) lysosomal granules and contain myeloperoxidase (MPO), lysosomal hydrolases, and lysosomal-type enzymes. They stain positive with MPO and Sudan black B.

**Clinical significance:** - Indicate myeloid lineage, so they are essentially diagnostic of AML (or a myeloid neoplasm such as high-grade MDS/CMML). They are not seen in ALL, which is MPO-negative and TdT-positive. - Present in the minority of AML cases overall, but a single Auer rod is enough to call a blast myeloid.

**Key associations:** - **APL (AML-M3):** t(15;17) creates the PML-RARα fusion, which blocks maturation at the promyelocyte stage. Cells contain abundant granules and numerous Auer rods, sometimes in bundles ("faggot cells"). Treatment is all-trans retinoic acid (ATRA, a vitamin A derivative that induces differentiation) plus arsenic trioxide. Complications are DIC (release of granule contents/tissue factor) and differentiation syndrome with ATRA. - **AML with maturation (M2)** and **AML with t(8;21) (RUNX1-RUNX1T1):** also frequently show Auer rods, and t(8;21) has large granules with Auer rods and carries a favorable prognosis. - **Monocytic AML (M4/M5):** Auer rods are less common. - Also seen in MDS with excess blasts, and rarely in CML blast crisis (myeloid).

**Board pearls:** - Auer rods = myeloperoxidase-positive = myeloid blasts = AML. - Pediatric ALL: no Auer rods; TdT+, CD10+, CD19+. - Age: AML typically in adults (median about 65); Auer rods plus DIC suggests APL, a medical emergency requiring immediate ATRA even before genetic confirmation. - Differentiation: Auer rods vs. Döhle bodies (blue cytoplasmic RNA remnants in neutrophils in infection) vs. Howell-Jolly bodies (nuclear remnants in RBCs).