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 of early differentiation may also help
  
• Potential difficulties may occur where blast cells have significant differentiation to more mature forms their primitive nature may be less easy to demonstrate although these cells are still those of acute leukaemia. This is most frequently encountered in monocytic cases of AML, or in acute promyelocytic leukaemia (APL)
  
  

For detailed description please see the link page below:
Tables of diagnostic markers supporting assignment of primitive phenotype in AML

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2. Myeloid lineage must be assigned

Like Auer rods or granulation in morphology, 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):

Pattern A: AML diagnosis based on lineage-defining markers
1. A myeloid lineage-defining marker pattern is present 2. No lineage-defining markers of T or B cells are present
Pattern B: AML diagnosis based on lineage-associated marker patterns
1. At least two myeloid lineage-associated markers are present 2. There are no lineage defining markers of T or B cells 3. No more than one T-cell or B-cell lineage-associated marker is present

For detailed description please see the link page below:
Tables of diagnostic markers supporting lineage assignment in AML

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3. Alternative diagnoses should be considered and excluded

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 (MPAL)
Consider MPAL if: you are able to assign myeloid lineage but the cells also express markers that meet the criteria for T or B cell lineage.
Acute Undifferentiated Leukaemia (AUL)
Consider AUL if: There is insufficient evidence to assign myeloid lineage, and you cannot assign T-cell or B-cell lineage either
Acute Leukaemia of ambiguous lineage not otherwise sepcified (ALAL-NOS)
Consider ALAL-NOS if: classification to a single lineage is not possible, but cells cannot be classed as AUL or MPAL.
Early T-cell precursor ALL
The marker pattern is of a primitive T cell neoplasm (cCD3 is expressed) but there are limited additional T markers, and at least one myeloid or stem cell marker is also expressed
Blastic plasmacytoid dendritic cell neoplasm (BPDCN)
Cases generally have skin rash and may resemble AUL or even AML; CD33 is frequently expressed but other markers are less frequently found while MPO and CD34 should be absent. Specific evidence should be sought if clinical features fit and initial makers are consistent.

For detailed description please see the link page below:
Tables of diagnostic patterns associated with aberrancy or alternative diagnosis in AML

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