The flow cytometric diagnosis of AML
From haematologyetc.co.uk
| 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
Click for more detailed description of identifying primitive immunophenotype in AML - 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)
Click for a more detailed description of problem areas in identifying primitive immunophenotype
| 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):
| Criteria to assign an AML diagnosis based on lineage-defining marker patterns |
|---|
| (1) A myeloid lineage-defining marker pattern is present (2) No lineage-defining markers of T or B cells are present Review details of these criteria |
| Criteria to assign an 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) There are no more than one T-cell or B-cell lineage-associated marker present Review details of these criteria |
Click for table of extended marker set for AML immunophenotype Myeloid lineage-defining markers Myeloid lineage-associated markers
Tables of diagnostic for lineage assignment in AML
| 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. ---- Click here for detailed description of MPAL diagnosis---- | |
| 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 ----Click here for detailed description of AUL diagnosis---- | |
| 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. ----Click here for detailed description of ALAL-NOS use---- | |
| 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 ----Click here for detailed description of ETP-ALL | |
| 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. ---- Click here for detailed description of BPDCN diagnosis---- |