The flow cytometric diagnosis of AML


1. The immunophenotype of the blast cells should be consistent with their "primitive" nature

AML M1.png

Assigning primitive nature in most cases is straightforward, but some types of AML may present difficulties.

Demonstrating primitive phenotype in AML
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
However, patterns are not always typical and difficult cases other markers of early differentiation may also help

Click for a more detailed table of markers associated with primitive phenotype
Difficulties may occur where blast cells have significant maturation so their primitive nature may be less easy to demonstrate.
Difficulties are most frequently encountered in monocytic cases of AML, or in acute promyelocytic leukaemia (APL) (although occasionally in other types).

Click to see common patterns that may cause difficulty in assigning primitive phenotype

2. The immunophenotype of the blast cells should allow myeloid lineage to be assigned

AML M2.png

The criteria to assign myeloid lineage in AML have been established, alternative sets of diagnostic criteria may be used (although many cases will meet more than one criteria set):

Pattern A: AML - diagnosis based on lineage-defining markers
A myeloid lineage-defining marker pattern is present and no lineage-defining markers of T or B cells are present

Click to view table of criteria

Pattern B: AML - diagnosis based on myeloid-lineage-associated marker patterns
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 to view table of criteria

Pattern C: AML - diagnosis based on less frequent differentiation patterns
In those rarer cases of erythroid or megakaryocytic differentiation, full myeloid criteria may not be met requiring these patterns to be specifically diagnosed.

Click to view table of criteria

3. Alternative diagnoses in difficult cases


In some cases myeloid markers may be present but full lineage may be unclear - in such cases it is important to consider possible alternative diagnoses

Alternative potential diagnoses in difficult cases with myeloid marker expression:

Mixed Phenotype Acute Leukaemia (MPAL)
Consider where myeloid lineage can be assigned based on lineage-specific patterns but the cells also have marker patterns that meet the criteria to assign 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: if classification to specific lineage is not possible but blast cells cannot be classed as AUL or MPAL.

This is most often a useful provisional diagnosis - click here for details
Early T-cell precursor acute lymphoblastic leukaemia (ETP-ALL)
This disorder has diagnostic criteria sufficient to assign T cell lineage (cCD3 is expressed) but may express myeloid-associated antigens cases may share features with MPAL M/T

Click for diagnostic criteria of ETP-ALL
Blastic plasmacytoid dendritic cell neoplasm (BPDCN)
Consider BPDCN: Generally in cases that resemble AUL (rarely AML), most often with skin rash. A specific marker profile should be sought: expression of bright CD4 and/or CD56 is expected. CD33 is frequently expressed but other myeloid markers are less frequent and MPO and CD34 should be absent. Look for specific additional markers as described in the diagnostic criteria.

Click for diagnostic criteria of BPDCN

FINAL NOTE Some "non-lineage" markers are frequently expressed in AML and may be associated with specific AML subtypes (Click here for further detail). Other features should give concern for alternative diagnosis (see the table below more detailed guidance).