Which cytochemical stain will be positive in blasts from patients with leukemias classified as French-American-British (FAB) M5?

Nonspecific Esterase (Acetate or Butyrate) is Positive in FAB M5 Leukemia Blasts

The correct cytochemical stain that will be positive in blasts from patients with leukemias classified as FAB M5 is nonspecific esterase (acetate or butyrate) (option E).

Cytochemical Staining in Acute Myeloid Leukemia Classification

Acute myeloid leukemia (AML) subtype M5, also known as acute monocytic leukemia, is characterized by a predominance of monoblasts and monocytes. The proper identification of this subtype relies on specific cytochemical staining patterns:

Nonspecific Esterase in Monocytic Leukemias

• Nonspecific esterase (NSE) using alpha-naphthyl acetate or butyrate as substrate shows diffuse cytoplasmic activity in monoblasts and monocytes 1
• In FAB M5 leukemia, NSE stains typically show strong positivity in approximately 80% of monoblasts 1
• The staining pattern may appear as either diffuse fine granular cytoplasmic staining or dense focal staining superimposed on a weak positive cytoplasmic background 2

Distinguishing Features of FAB M5

• FAB M5 (acute monocytic leukemia) is characterized by:
  • ≥80% of cells in the bone marrow being of monocytic lineage
  • Strong positivity for nonspecific esterase stains
  • Monoblasts and promonocytes counted as blast equivalents 1

Other Cytochemical Stains and Their Significance

Why Other Options Are Incorrect:

1. TdT (Terminal Deoxynucleotidyl Transferase) - Option A

   • Typically positive in lymphoid malignancies, not in myeloid leukemias
   • Used to identify lymphoblastic leukemias, not monocytic leukemias

2. Specific Esterase (Chloroacetate) - Option B

   • Positive in granulocytic lineage (FAB M1, M2, M3)
   • Negative in monocytic lineage (FAB M5)

3. Tartrate Resistant Acid Phosphatase (TRAP) - Option C

   • Characteristic of hairy cell leukemia
   • Not typically positive in AML subtypes

4. Leukocyte Alkaline Phosphatase - Option D

   • Elevated in chronic myeloid disorders and infections
   • Not a characteristic marker for acute monocytic leukemia

Diagnostic Approach to FAB M5

The diagnosis of FAB M5 relies on a combination of:

1. Morphology: Presence of monoblasts and promonocytes
2. Cytochemistry: Strong positivity for nonspecific esterase (NSE) stains
3. Immunophenotyping: Expression of monocytic markers (CD11c, CD14, CD64, lysozyme, CD4, CD11b, CD36) 1

Clinical Significance

• FAB M5 is associated with specific clinical features including:
  • Higher risk of extramedullary involvement
  • Potential CNS involvement requiring screening lumbar puncture 1
  • May be associated with 11q23 (MLL) abnormalities which often express the NG2 homologue 1

Pitfalls in Diagnosis

• Some B-lineage acute lymphoblastic leukemias can show aberrant NSE positivity, potentially causing confusion with monocytic leukemia 3
• This highlights the importance of using multiple diagnostic modalities including immunophenotyping to confirm lineage
• Flow cytometry is now considered the gold standard for lineage determination, but cytochemistry remains valuable in resource-limited settings 4

The use of nonspecific esterase staining remains an important diagnostic tool in the classification of acute leukemias, particularly in identifying the monocytic component in FAB M5 leukemia, with a high concordance rate (97%) between NSE staining and other diagnostic methods 5.