## Confirming ALL: Flow Cytometry as the Diagnostic Gold Standard **Key Point:** Flow cytometry with B- and T-cell markers (CD19, CD20, CD10, CD7, CD5, TdT) is the most specific investigation for confirming acute lymphoblastic leukemia (ALL) and is essential for: - Lineage assignment (B-cell vs T-cell ALL) - Immunophenotypic subclassification (e.g., common ALL, pre-B ALL, T-ALL) - Prognostic stratification (CD10+ vs CD10−, TdT expression) - Detection of minimal residual disease (MRD) during follow-up ## Cytochemical Stains in Acute Leukemia Diagnosis | Stain | AML | ALL | Specificity | |---|---|---|---| | **MPO (Myeloperoxidase)** | **Positive** | Negative | Myeloid lineage marker | | **PAS (Periodic acid-Schiff)** | Positive (variable) | **Positive (blocks/granules)** | Glycogen accumulation | | **Sudan Black** | Positive | Negative | Lipid in myeloid granules | | **Flow cytometry** | Myeloid markers (CD13, CD33) | **B/T markers (CD19, CD7, TdT)** | **Most specific for lineage** | **High-Yield:** In ALL, cytochemical stains are often **negative or weakly positive**, making them unreliable for diagnosis. Flow cytometry is superior because it directly identifies lymphoid lineage markers and is the reference standard for ALL classification. **Clinical Pearl:** The morphology (small blasts, scant cytoplasm, high N:C ratio) and absence of Auer rods are consistent with ALL, but flow cytometry is mandatory to confirm lymphoid lineage and subtype the disease (common ALL, pre-B, T-ALL), which has prognostic and therapeutic implications. ## Why Flow Cytometry Is Superior for ALL 1. **Direct lineage identification:** CD19/CD20 (B-ALL), CD7/CD5 (T-ALL) 2. **Immunophenotypic subtyping:** Common ALL (CD10+), pre-B ALL (CD10−), T-ALL 3. **Prognostic markers:** CD10 status, TdT expression, CD34 (stem cell marker) 4. **MRD detection:** Flow cytometry can detect 1 leukemic cell in 10,000 normal cells
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