The flow cytometric diagnosis of AML

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1. Blast cells should have a "primitive" immunophenotype

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

• In most cases, cells of 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
  
• Difficulties may occur where blast cells have significant maturation where their primitive nature may be less easy to demonstrate. 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. The immunophenotype should allow myeloid lineage to 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, two alternative sets of criteria may be used (although most cases will meet both):

Pattern A: AML diagnosis based on lineage-defining markers
A myeloid lineage-defining marker pattern is present
No lineage-defining markers of T or B cells are present
Pattern B: AML diagnosis based on lineage-associated marker patterns
At least two myeloid lineage-associated markers are present
There are no lineage defining markers of T or B cells
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 alternative diagnosis (see the table below more detailed guidance).

Alternative potential diagnoses in difficult cases:

Mixed Phenotype Acute Leukaemia (MPAL)
Consider MPAL: Where myeloid lineage can be assigned based on lineage-specific or lineage-associated patterns and The cells are found to have marker patterns that meet the criteria to assign either T or B cell lineage. Click for diagnostic criteria of MPAL
Acute Undifferentiated Leukaemia (AUL)
Consider AUL In cases where the evidence is insufficient to assign myeloid lineage and There is insufficient evidence to assign to T-cell or B-cell lineage Click for diagnostic criteria of AUL
Acute Leukaemia of ambiguous lineage not otherwise sepcified (ALAL-NOS)
Consider ALAL-NOS where classification to specific lineage is not possible, but cells cannot be classed as AUL or MPAL. This is often helpful as a provisional diagnosis while additional evidence is sought
Early T-cell precursor acute lymphoblastic leukaemia (ETP-ALL)
This disorder has the features of a primitive T cell neoplasm (cCD3 is expressed). However there are limited additional T markers and at least one myeloid or stem cell marker is also expressed this can be difficult to distinguish from a T/myeloid MPAL
Blastic plasmacytoid dendritic cell neoplasm (BPDCN)
A skin rash is typical. Cases may resemble AUL (or even AML). CD33 is frequently expressed but other myeloid markers are less frequently found and MPO and CD34 should be absent. Specific marker patterns will allow diagnosis.

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