Acute Leukemia of Ambiguous Lineage:
1. Entities excluded from this diagnosis:
- AML with t(8;21), inv(16) and PML-RARA fusion
- AML with myelodysplasia-related changes
- Therapy related AML
- Leukemia with FGFR1
- CML in blast crisis
2. Acute undifferentiated leukemia:
- Express no more than one membrane marker for any given lineage
- Lack expression of CyCD3, MPO, CyCD22, CyCD79 or CD19
- Lack features of cells of other lineages such as megakaryocytes and plasmacytoid dendritic cells
3. Mixed-phenotype acute leukemia (acute bilineage or bilineal leukemia):
- Separate populations of blasts of more than one lineage
- Requirement for lineage assignment is not as stringent as cases with single blast population
4. Mixed-phenotype acute leukemia (acute biphenotypic leukemia):
- Single population of blasts coexpressing antigens of more than one lineage (see Fig 1).
Requirement for assigning more than one lineage to a single blast population:
Myeloid lineage:
- MPO (by flow, IHC or cytochemistry); or
- ≥ 2 monocytic markers: non-specific esterase, CD11c, CD14, CD64 and lysozyme
T-cell lineage:
- Strong cytoplasmic CD3 (by flow); or surface CD3
- CD3 expression need to be strong: brightest cCD3+ blasts should reach the intensity of the normal T-cells (strong expression is not required for T-ALL diagnosis)
B-cell lineage:
- Strong CD19 with ≥1 of strongly expressed CD79a, CyCD22, CD10; or
- Weak CD19 with ≥2 of strongly expressed CD79a, CyCD22, CD10
Fig 1.Mixed-phenotype acute leukemia, B/myeloid. Blasts (red) are variably positive for B-cell markers (CD19, CyCD79a and CD22), positive for CD13 and partially positive for CD117 and MPO. T-cells (dark blue) can be used as negative control. Normal B-cells (green) can be used as positive control.
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