What are the peripheral blood smear findings in acute myelomonocytic leukemia (AML M4)?

Peripheral Smear Findings in Acute Myelomonocytic Leukemia (AML M4)

In acute myelomonocytic leukemia (AML M4), the peripheral blood smear shows a mixed population of myeloblasts and monoblasts/promonocytes, typically with both granulocytic and monocytic differentiation visible, and blast counts often exceeding 20% of circulating leukocytes.

Key Morphologic Features

The peripheral smear in AML M4 demonstrates:

• Dual lineage blast population: Both myeloblasts (with fine chromatin and Auer rods in some cases) and monoblasts/promonocytes (with more abundant cytoplasm and folded nuclei) 1
• Blast percentage: Typically ≥20% of peripheral blood leukocytes, though the exact percentage varies 1
• Promonocytes: These are counted as blast equivalents in AML M4, distinguishing it from other AML subtypes. Importantly, mature abnormal monocytes are NOT counted as blasts 1
• Auer rods: May be present in the myeloid component, though "faggot-like" bundles of Auer rods are uncommon in M4 and more characteristic of M3 (APL) 2

Cytochemical Staining Patterns

When cytochemistry is performed (still used in some countries over flow cytometry):

• Myeloperoxidase (MPO) or Sudan Black B (SBB): Positive in ≥3% of blasts, indicating myeloid differentiation 1
• Nonspecific esterase (NSE): Shows diffuse cytoplasmic positivity in monoblasts (usually 80% positive) and monocytes (usually 20% positive) 1
• The combination of MPO positivity AND NSE positivity in different blast populations is characteristic of the myelomonocytic lineage

Immunophenotypic Markers

Flow cytometry typically reveals:

• Granulocytic markers: CD13, CD33, cytoplasmic MPO 1
• Monocytic markers: CD11c, CD14, CD64, lysozyme, CD4, CD36 1
• Precursor markers: CD34, CD117, HLA-DR (variable) 1
• Evidence of monocytic differentiation requires at least 2 of the following: NSE, CD11c, CD14, CD64, or lysozyme 1

Important Diagnostic Considerations

Common pitfalls to avoid:

1. Do not count mature monocytes as blasts – only monoblasts and promonocytes count toward the blast percentage in M4 1

2. Eosinophilia may be present – particularly in cases with inv(16) or t(16;16), which can show abnormal eosinophils with basophilic granules 3, 4

3. Peripheral blood alone may suffice for diagnosis if adequate blasts are present (≥20%), though bone marrow examination remains the gold standard 5, 1

4. Aberrant lymphoid marker expression can occur (e.g., CD19) and does not exclude AML M4 diagnosis 4

Diagnostic Workflow

Per ASCO/CAP/ASH guidelines, the pathologist should 5:

1. Review CBC counts and leukocyte differentials
2. Evaluate peripheral blood smear with at least 200 leukocytes counted 1
3. Perform bone marrow aspirate with 500 nucleated cells counted (when feasible) 1
4. Obtain flow cytometry to confirm dual myeloid/monocytic differentiation 5
5. Perform conventional cytogenetics (mandatory) to identify recurrent abnormalities like inv(16) or t(16;16) 5, 1

The diagnosis requires integration of morphology showing both myeloid and monocytic differentiation, with confirmation by cytochemistry or immunophenotyping demonstrating markers of both lineages 1.