The flow cytometric diagnosis of AML

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1. The primitive nature of the blast cells should be demonstrated

The "primitive" morphology of blast cells is accompanied by signs of primitive immunophenotype

• in most cases AML will demonstrate typical features of immature cells with: weak expression of CD45, and expression of CD34 and/or CD117. In difficult cases other markers may help (see Click for more detailed description)
• NOTE In cases where blast cells show differentiation to more mature forms their primitive nature may be less clear. This is most frequently encountered in monocytic cases of AML, or in acute promyelocytic leukaemia (APL) (Click for a more detailed description)

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2. Can you assign myeloid lineage?

Like Auer rods or granulation in morphology, particular immunophenotypic features support assignment to myeloid lineage

The criteria to assign myeloid lineage in AML have been established by the WHO, two alternative sets of criteria may be used (although most cases will meet both):

Criteria Set 1:
A myeloid lineage-defining marker pattern is present and no lineage-defining markers of T or B cells are present (Click for detailed description)
Criteria Set 2:
At least two myeloid lineage-associated markers are present and there are no lineage defining markers of T or B cells and no more than one: T-cell or B-cell lineage-associated marker is present (Click for detailed description)

NOTE In some cases an extended marker panel may allow the pattern of differentiation to be identified (e.g. erythroid or megakarocytic). In difficult cases these markers may provide additional evidence of myeloid lineage (although their specificity is often less than the general myeloid-associated markers so care is required when using them in this way).

Click for table of extended marker set for AML immunophenotype

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3. Should alternative diagnoses be considered?

In some cases lineage may be unclear - either because myeloid lineage cannot be confidently assigned, or because markers of other lineage are present - in such cases it is important to consider possible alternative diagnoses

• NOTE Some "non-lineage" markers are frequently expressed in AML and may be associated with specific AML subtypes, these do not necessarily indicate mixed phenotype (Click here for further detail)
• Other features should give concern for mixed phenotype or alternative diagnosis, it is important in such case that diagostic criteria and alternative diagnoses are carefully considered (see the table below more detailed guidance).

Table: Possible alternative diagnoses in difficult cases

Mixed Phenotype Acute Leukaemia
Consider MPAL if: you are able to assign myeloid lineage but the cells also express markers that fit the criteria for T or B cell lineage. See: MPAL
Acute Undifferentiated Leukaenmia
Consider AUL if: There is insufficient evidence to assign myeloid lineage, and you cannot assign T-cell or B-cell lineage either See AUL
Acute Leukaemia of ambiguous lineage not otherwise sepcified
Consider AL-NOS if: where definitive classification along a single lineage is difficult, but markers do not allow classification as either acute undifferentiated leukemia, or mixed-phenotype acute leukemia. See ALAL-NOS
Non-haematological malignancy
Consider non-haematological malignancy particularly where CD45 expression is absent or very weak and the presentation is atypical. Particularly rememeber some markers associated with acute leukaemia may be expressed by carcinoma See Non-haematological malignancy
Early T-cell precursor ALL
The marker pattern is of a primitive T cell neoplasm with lineage-defining criteria, but there are also myeloid markers expressed See: ETP-ALL
Blastic plasmacytoid dendritic cell neoplasm
Morphologically BPDCN may resemble AML (particularly monocytic), frequently with skin accompanying rash. Identification may require additional markers See: BPDCN

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