Flow cytometry:ETP-ALL

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

The immunophenotype of ETP-ALL requires careful consideration – this reflects that the condition was identified using gene expression profiling rather than by immunophenotype. Using immunophenotype to establish the diagnosis is therefore challenging and may underestimate the number of true cases.* and may not be easily separated from ALAL T/my or T-ALL

To diagnose ETP-ALL the cells need to meet specific criteria. These are given in the table below:

Requirement 1

1. T-lineage should be demonstrated: (may be heterogenous but should be expressed by ≥25% of blasts)Assignment of T-Lymphoid lineage in ETP-ALL 2. One or more myeloid antigen (CD11b, CD13, CD33, CD65, CD117) and/or stem cell antigens (CD34, HLA-DR) should be present 3. Note that CD7 is consistently positive in ETP-ALL and does not count as a stem cell antigen in this context

Requirement 2

*Required absence: CD3, CD1a and CD8 (<5% of blasts) Required absent/dim CD5 expression (<75% positive blasts)

NOTES

CD4 may have the same pattern The ETP-ALL immunophenotype may also be established by immunohistochemistry. Distinction from mixed-phenotype acute leukaemia (MPAL) requires that MPO is not expressed - in this context of ETP-ALL the WHO advocate a threshold of <3% to define negative myeloperoxidase expression (by cytochemistry or flow cytometry). This threshold is different from that of T/myeloid MPAL so may not be optimal but is retained at present. T-ALL cases that have an immunophenotype similar to ETP-ALL but where CD5 is expressed (≥75% of blasts) may be designated as “near-ETP-ALL”, but the clinical implications of such a designation remain unclear.