## Clinical Context This patient has CML in blast crisis (accelerated phase progressing to acute leukemia), evidenced by sudden appearance of blasts with Auer rods on peripheral smear. The presence of Auer rods is pathognomonic for acute myeloid leukemia (AML) and indicates transformation from chronic phase. ## Why Bone Marrow Aspiration & Biopsy is the Investigation of Choice **Key Point:** Bone marrow aspiration and biopsy with cytochemical stains (myeloperoxidase, Sudan black B) is the gold standard for: - Confirming blast percentage (>20% defines AML) - Determining lineage of blasts (myeloid vs. lymphoid) - Identifying morphologic features (Auer rods, dysplasia) - Assessing cellularity and fibrosis **High-Yield:** Cytochemical stains are essential because: - **MPO (myeloperoxidase) positive** = myeloid lineage (AML) - **Sudan black B positive** = lipid in myeloid blasts - These stains differentiate AML from ALL when morphology is ambiguous ## Why Other Investigations Are Insufficient as First-Line | Investigation | Limitation in This Context | |---|---| | Flow cytometry | Confirms immunophenotype but does NOT assess morphology or cytochemistry; cannot be sole diagnostic test | | RT-PCR for BCR-ABL1 | Confirms CML clone but does NOT diagnose blast crisis or AML transformation; BCR-ABL1 may persist in blast phase | | Cytogenetics for t(9;22) | Confirms CML origin but does NOT diagnose acute leukemia; t(9;22) persists in blast crisis | **Clinical Pearl:** Blast crisis in CML can be AML (70%) or ALL (30%). Bone marrow morphology + cytochemistry determines lineage and guides therapy (AML vs. ALL protocols). **Warning:** Do not rely on peripheral smear alone — bone marrow is needed to assess blast percentage (diagnostic threshold ≥20% for AML) and rule out other causes of circulating blasts.
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