Which cytochemical stain is most useful for distinguishing acute promyelocytic leukemia (APL) from other acute myeloid leukemias?

Question

Accepted Answer

B. Myeloperoxidase (MPO) stain. ## Cytochemical Stains in Acute Leukemias

**Key Point:** MPO stain is the gold standard for identifying myeloid differentiation and is strongly positive in APL, helping distinguish it from other AML subtypes and ALL.

### Why MPO for APL?
APL (AML-M3) is characterized by abnormal promyelocytes with abundant Auer rods and heavy cytoplasmic granulation. The MPO stain highlights the myeloid lineage commitment and is intensely positive in these cells, confirming myeloid origin and aiding morphologic diagnosis.

### Comparative Utility of Stains

| Stain | APL | Other AML | ALL | Utility in APL |
| --- | --- | --- | --- | --- |
| MPO | +++ (strong) | ++ to +++ | − | **Best for confirming myeloid lineage** |
| Sudan Black B | +++ | ++ to +++ | − | Similar to MPO; parallel results |
| PAS | Variable | ++ | +++ | Non-specific; not discriminatory |
| TRAP | − | − | − (except hairy cell) | Not useful for APL |

**High-Yield:** Both MPO and Sudan Black B are equally sensitive for myeloid differentiation, but MPO is more widely used and standardized in modern practice. APL is unique among AMLs for its **strong, diffuse MPO positivity** and characteristic morphology (abundant Auer rods, heavy granulation).

**Clinical Pearl:** In APL, the combination of strong MPO positivity + abnormal promyelocytes with multiple Auer rods ("faggot cells") is virtually pathognomonic. This morphologic-cytochemical correlation is crucial for rapid diagnosis, as APL is a medical emergency requiring immediate ATRA and arsenic trioxide therapy.

Answer

A. Tartrate-resistant acid phosphatase (TRAP) stain

Answer

C. Periodic acid-Schiff (PAS) stain

Answer

D. Sudan Black B stain

Which cytochemical stain is most useful for distinguishing acute promyelocytic leukemia (APL) from other acute myeloid leukemias?

Explanation

Cytochemical Stains in Acute Leukemias

Why MPO for APL?

Comparative Utility of Stains

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Stain	APL	Other AML	ALL	Utility in APL
MPO	+++ (strong)	++ to +++	−	Best for confirming myeloid lineage
Sudan Black B	+++	++ to +++	−	Similar to MPO; parallel results
PAS	Variable	++	+++	Non-specific; not discriminatory
TRAP	−	−	− (except hairy cell)	Not useful for APL