A 5-year-old boy is found to have 92,000 WBC/μL with 85% blasts on CBC. Blood smear shows small, round blasts with scant cytoplasm and high nuclear-to-cytoplasmic ratio. No Auer rods are seen. Which investigation is most specific for confirming acute lymphoblastic leukemia and determining prognosis?

Question

Accepted Answer

C. Flow cytometry with B- and T-cell markers. ## 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

Answer

A. Periodic acid-Schiff (PAS) stain on bone marrow

Answer

B. Sudan Black stain on bone marrow

Answer

D. Myeloperoxidase (MPO) stain on bone marrow

Explanation

Confirming ALL: Flow Cytometry as the Diagnostic Gold Standard

Cytochemical Stains in Acute Leukemia Diagnosis

Why Flow Cytometry Is Superior for ALL

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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