A 52-year-old man from Delhi presents with progressive weakness, bone pain, and recurrent infections over 4 months. On examination, he has hepatosplenomegaly and generalized lymphadenopathy. Laboratory investigations show hemoglobin 9.8 g/dL, WBC 68,000/μL (with 65% blasts), and platelets 45,000/μL. Peripheral blood smear reveals numerous large blasts with abundant cytoplasm, prominent nucleoli, and Auer rods. Cytochemical staining shows positive myeloperoxidase (MPO) and positive Sudan black stain. What is the most likely diagnosis?

Explanation

## Diagnosis: Acute Myeloid Leukemia (AML) ### Pathognomonic Smear Findings **Key Point:** Auer rods are the hallmark of AML and are virtually diagnostic of myeloid differentiation. They are abnormal fusion products of azurophilic granules and are NOT seen in ALL or other hematologic malignancies. ### Peripheral Blood Smear Morphology in AML | Feature | Finding in This Case | Diagnostic Value | |---------|----------------------|------------------| | **Auer rods** | Present | Pathognomonic for AML; absent in ALL | | **Blast size** | Large | Consistent with AML | | **Cytoplasm** | Abundant | Myeloid blasts typically have more cytoplasm than lymphoid | | **Nucleoli** | Prominent | Reflects high transcriptional activity | | **Granules** | Visible (Sudan black+) | Indicates myeloid lineage | | **MPO staining** | Positive | Confirms myeloid differentiation | **High-Yield:** Auer rods are present in ~60% of AML cases and are considered a defining feature. Their presence alone is sufficient to diagnose AML, even without flow cytometry or immunophenotyping. ### Cytochemical Staining Pattern in AML ```mermaid flowchart TD A[Acute Leukemia]:::outcome --> B{MPO positive?}:::decision B -->|Yes| C[Myeloid lineage]:::action B -->|No| D[Lymphoid lineage]:::action C --> E{Auer rods?}:::decision E -->|Yes| F[AML - Confirmed]:::action E -->|No| G[AML - Likely]:::action D --> H[ALL - Likely]:::action C --> I{Sudan black+?}:::decision I -->|Yes| J[Confirms myeloid origin]:::action ``` **Mnemonic:** **MPO-positive + Auer rods = AML** ### Cytochemical Stain Interpretation | Stain | AML | ALL | Significance | |-------|-----|-----|-------------| | **Myeloperoxidase (MPO)** | Positive (>3%) | Negative | Gold standard for myeloid differentiation | | **Sudan Black** | Positive | Negative | Detects lipid content in myeloid granules | | **PAS (Periodic Acid-Schiff)** | Negative/weak | Positive (coarse blocks) | Carbohydrate in lymphoid blasts | | **Acid Phosphatase** | Negative/diffuse | Positive (focal) | Helps distinguish AML-M5 from ALL | **Clinical Pearl:** In this case, BOTH MPO and Sudan black are positive, providing redundant confirmation of myeloid lineage. This is highly specific for AML. ### Why This Is AML and Not Other Diagnoses **Key Point:** The combination of Auer rods + MPO positive + Sudan black positive is diagnostic of AML. No other acute leukemia shows this pattern. ### AML Subtypes (FAB Classification) While the question does not specify the FAB subtype, the presence of Auer rods and abundant cytoplasm is consistent with: - **AML-M1 (without maturation):** Myeloblasts, few Auer rods - **AML-M2 (with maturation):** Myeloblasts + maturing forms, frequent Auer rods ← **Most common** - **AML-M3 (Promyelocytic):** Abnormal promyelocytes, abundant Auer rods ("faggot cells") **High-Yield:** AML-M2 is the most common AML subtype (~30% of cases) and frequently shows Auer rods on smear. ### Clinical Context - **Age 52:** AML incidence increases with age (median age ~68 years) - **Hepatosplenomegaly + lymphadenopathy:** Leukemic infiltration - **Anemia, thrombocytopenia, infection:** Bone marrow failure from leukemic replacement - **WBC 68,000/μL with 65% blasts:** Diagnostic of acute leukemia (>20% blasts = AML) [cite:Robbins 10e Ch 13; Wintrobe's Hematology 14e Ch 91]