The flow cytometric diagnosis of AML

Acute leukaemia types

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1. The immunophenotype of AML blasts will (generally) reflect their primitive nature

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

• Most often in AML typical features of immature cells will be found with: weak expression of CD45, together with expression of CD34 and/or CD117. Other markers may be useful in difficult cases (see Click for more detailed description)
• However in cases where blast cells show more differentiation their nature may be less clear. This is most frequently encountered in monocytic cases of AML or in APL (Click for a more detailed description)

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2. The cases should express sufficient markers to allow myeloid lineage to be assigned

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

The requirements to diagnose AML by flow cytometry have been established by WHO, cases should meet either of two alternaitive criteria (most 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 detaied 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 markers are present

NOTE In cases with atypical maturation an extended marker panel may be used to allow the differentiationto pattern be identified (e.g. erythroid or megakarocytic). These markers may also be used in difficult cases to provide additional evidence of myeloid lineage. However the specificity of these markers is often less than the general myeloid-associated markers so care is required when using them in this way.
(see Table for details of the extended marker set)

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3. Are atypical features present? Should alternative diagnoses be considered?

Difficult cases often arise where 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

• The expression of "non-lineage" markers in AML is well recognised, and in many cases these should be simply regarded as "expected aberrancy" while others are associated with specific AML subtypes, these do not necessarily indicate mixed phenotype, but when these markers are detected then this requires specific consideration (Click here for further detail)
• Some features should give concern for mixed phenotype or alternative diagnosis. While these circumstances are infrequent, it is important that where immunophenotype is atypical then the criteria should be carefully reviewed (see the table below more detailed guidance).

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Possible alternative diagnoses in difficult cases:

1. Mixed Phenotype Acute Leukaemia. See: MPAL
Consider MPAL if: you are able to assign myeloid lineage but the cells also express either a lineage-defining marker of T or B cells, or more than one lineage associated marker of T or B cells.
2. Acute Undifferentiated Leukaenmia: See AUL
Consider AUL if: There is insufficient evidence to assign myeloid lineage, and you cannot assign T-cell or B-cell lineage.
3 Non-haematological malignancy: See Non-haematological malignancy
Consider non-haematological malignancy if: myeloid markers may be expressed but not conclusice, CD45 expression is very weak, and the presentation is atypical.
4. Early T-cell precursor ALL: See: ETP-ALL
The marker pattern is of a primitive T cell neoplasm with lineage-defining criteria, but there are also myeloid markers expressed
5. Blastic plasmacytoid dendritic cell neoplasm: See: BPDCN
Morphologically BPDCN may resemble AML (particularly monocytic), frequently with skin accompanying rash. Identification may require additional markers

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