Flow Cytometry Side Scatter / CD45 Blast Window

The blast window refers to the area on the SSc/CD45 plot which is SSc-dim/CD45-dim. In normal samples, it should be relatively unoccupied.

Differentials for a cell population in the blast window include:

  • Leukaemic blasts (both myeloid and lymphoid)
  • Lymphoma cells
  • Hypogranular promyelocytes (seen in the hypogranular variant of acute promyelocytic leukaemia)
  • Plasma cells
  • Basophils
  • Plasmacytoid dendritic cells
The blast window is the area of the side scatter vs CD45 plot which is SSc-dim and CD45-dim. As illustrated, it is usually relatively empty.

Blast Evolution and Maturation

As common progenitors become committed, their side scatter and CD45 properties change to resemble their mature counterparts. The image below shows how SSc and CD45 change with blast maturation.

Schematic representation of how side scatter and CD45 intensity changes as blasts mature and become more lineage-committed.

Note that blast ennumeration by flow cytometry may not tally with morphological counts. There are a number of reasons for this, including:

  • RBC lysing removes variable number of erythroid precursors
  • Variable degree of peripheral blood dilution, affecting blast estimate
  • Morphological characterisation of blasts may differ from immunotypic characterisation (for example, promonocytes are counted as blasts in morphological assessment, but may be differentiated from monoblasts on flow cytometry).

Immunophenotype of CD34+ Lineage-Committed Blasts

LineageImmunophenotype
NeutrophilSSc high / CD45 dim / 13++ / MPO+
EosinophilSSc high / CD45+ / 15+ / cytoplasmic EPO+
BasophilSSc low / CD45+ / HLA-DR dim / 117 dim / 123++ / 203c+
MonocyteSSc intermediate / CD45+ / HLA-DR+ / 117 dim / 64+
ErythroidSSc intermediate / CD45 dim / 36+ / 105+
Mast CellSSc intermediate or high / CD45++ / 117++ / HLA-DR dim
Plasmacytoid Dendritic CellSSc intermediate / CD45 dim / HLA-DR++ / 123++ / 36+
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