## Distinguishing AML from ALL: Cytochemical and Morphologic Markers ### Key Discriminators **Key Point:** Auer rods (pathognomonic for AML) and myeloperoxidase (MPO) positivity are the gold-standard morphologic and cytochemical features that definitively separate AML from ALL. ### Cytochemical Comparison | Feature | AML | ALL | |---------|-----|-----| | **Auer rods** | Present (pathognomonic) | Absent | | **MPO stain** | Strongly positive | Negative | | **PAS stain** | Negative or weak | Positive (block pattern) | | **Sudan black** | Positive | Negative | | **Butyrate esterase** | Positive (monocytic) | Negative | **High-Yield:** Auer rods are diagnostic of AML and, when present, make the diagnosis certain regardless of other findings. They represent abnormal fusion of primary granules and are never seen in ALL. ### Morphologic Features of AML 1. **Cytoplasm:** Abundant, basophilic, often with azurophilic granules 2. **Nucleus:** Large, irregular, with prominent nucleoli 3. **Auer rods:** Needle-like or rod-shaped inclusions (fusion of Auer bodies) 4. **MPO reactivity:** Indicates myeloid lineage differentiation ### Why Auer Rods Matter **Clinical Pearl:** Auer rods are so specific for AML that their presence alone can distinguish AML from ALL even without immunophenotyping. They are associated with acute promyelocytic leukemia (APL/AML-M3) and other AML subtypes. **Mnemonic:** **AUER = AML's Unique Electron-dense Rods** — a pathognomonic finding that clinches the diagnosis. ### Immunophenotypic Confirmation While cytochemistry is rapid, flow cytometry confirms: - **AML:** CD13, CD33, CD117 (myeloid antigens); MPO+ - **ALL:** CD19, CD20, CD10 (B-cell) or CD7, CD5 (T-cell); MPO−, TdT+ [cite:Robbins 10e Ch 24] 
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