Auer rods in a B/myeloid-mixed phenotype acute leukemia

A 21-year-old female presented with fatigue and fever for 2 weeks. On examination, she had ecchymosis over trunk and limbs. Complete blood count revealed anemia, thrombocytopenia, and leukocytosis. Peripheral blood smear showed 85% blasts, many with Auer rods and myeloperoxidase (MPO) positivity [Figure 1a-c], May-Grunwald-Giemsa × 1000). On flow cytometry immunophenotyping [Figure 1d], blasts were positive for myeloid (CD13, CD33, CD117, and anti-MPO) and B-lineage (CD19, cytoCD79a, and cytoCD22) markers. On molecular genetics analysis, breakpoint cluster region-abelson (BCR::ABL1) fusion gene and mixed lineage leukemia (MLL) gene rearrangement were negative; however, FMS-like tyrosine kinase 3 internal tandem duplication (FLT3-ITD) mutation was positive. A diagnosis of mixed phenotype acute leukemia (MPAL) (B-lymphoid/myeloid) was made.

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Figure 1:

(a-c) Peripheral smear showing blasts with Auer rods and myeloperoxidase (inset in b) positivity (x1000; May Grunwald-Giemsa). (d) Flow cytometry dot plots showing blasts positive for myeloid (CD13, CD33, CD117, anti-MPO) and B-lineage (CD19, cytoCD79a and cytoCD22) markers.

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Auer rods are needle-like structures formed by agglomeration of azurophilic granules. Identification of Auer rods in the blasts is considered as one of morphological features of myeloblasts and is diagnostic for acute myeloid leukemia (AML). The presence of Auer rods in MPAL has been very rarely reported. Auer rods are more commonly seen in myeloid/T-lymphoid than myeloid/B-lymphoid subtype of MPAL.

The diagnostic learning in our case is that although the detection of Auer rods does confirm the myeloid lineage and warrant a diagnosis of AML, it is still imperative to perform flow cytometric analysis for exclusion of MPAL.