A 4-year-old boy presents with a 2-week history of fever, pallor, and petechiae. On examination, he has hepatosplenomegaly and generalized lymphadenopathy. Complete blood count shows hemoglobin 7.2 g/dL, WBC 85,000/μL with 60% blasts, and platelets 35,000/μL. Which is the investigation of choice to confirm the diagnosis and determine the subtype of leukemia?

Explanation

## Diagnostic Approach to Childhood Leukemia **Key Point:** Bone marrow aspiration and biopsy with comprehensive analysis (morphology, cytochemistry, and immunophenotyping) is the gold standard for confirming acute leukemia diagnosis and determining subtype (ALL vs AML). ### Why Bone Marrow Examination is Essential **High-Yield:** The diagnostic criteria for acute leukemia require ≥20% blasts in bone marrow or peripheral blood. While this patient's peripheral blood already shows 60% blasts (diagnostic), bone marrow examination provides: 1. **Morphologic assessment** — cell size, cytoplasmic features, nuclear-to-cytoplasmic ratio, and auer rods (pathognomonic for AML) 2. **Cytochemical stains** — myeloperoxidase (MPO) positive in AML, negative in ALL; PAS positivity in ALL 3. **Immunophenotyping** — B-cell vs T-cell lineage, stage of differentiation, and prognostic markers (CD34, CD10, etc.) 4. **Cytogenetics and molecular studies** — t(15;17) in APL, t(9;22) in CML-like ALL, hyperdiploidy, etc. ### Differential Diagnosis of ALL vs AML | Feature | ALL | AML | |---------|-----|-----| | MPO stain | Negative | Positive | | PAS stain | Positive (block pattern) | Negative or focal | | Auer rods | Absent | Present (diagnostic) | | CD34 | Often positive | Variable | | Common CD markers | CD10, CD19, CD20 | CD13, CD33, CD117 | **Clinical Pearl:** Even with high circulating blasts, bone marrow examination is performed because it provides the most comprehensive diagnostic and prognostic information, guiding treatment decisions (ALL-directed vs AML-directed therapy). **Mnemonic: BMAT** — **B**one marrow, **M**orphology, **A**ntigen (immunophenotyping), **T**iming (cytogenetics) — all needed for complete characterization. ### Role of Other Investigations - **Flow cytometry of peripheral blood alone:** While useful for immunophenotyping, it cannot reliably assess bone marrow cellularity, morphology, or cytochemistry; not sufficient as sole diagnostic test. - **Imaging (CXR, ultrasound):** Useful for assessing organomegaly and mediastinal involvement but does NOT confirm leukemia diagnosis. - **Lumbar puncture:** Performed AFTER diagnosis is confirmed to assess CNS involvement (prophylaxis/treatment planning), not for initial diagnosis. [cite:Park 26e Ch 10]