Atypical patterns of primitive marker expression in acute myeloid leukaemia

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Table: Less frequent patterns of primitive phenotype;
While blast cells typically cause low side scatter of light and have an expression of CD45 that is lower than normal lymphocytes or monocytes, atypical patterns may be encountered. The more frequent patterns are described below.
AML with monocytic differentiation	In AML with monocytic maturation those primitive cells with more mature monocytic features may show higher side scatter (consistent with their size and cytological features) and some or all of the blast cells may also have high expression of CD45 (comparable with normal or CMML monocytes). Such cases may be difficult to distinguish from typical or transforming CMML. It is important to compare with clinical and morphological features to ensure appropriate classification.
Acute promyelocytic leukaemia	Cells of APL can vary in size and granularity an will often express a range of side scatter features that may be be similar to maturing granulocytic cells, similarly CD45 expression may be similar to maturing granulocytic cells. The distribution of cells will be abnormal may include cells in the typical blast area, but the wide range of scatter features can make them difficult to identify as primitive cells of a reactive process or (in some cases)of cells of MDS. Morphological features are important, and consider the common marker pattern (including HLA-DR expression (see table).
Other patterns	Frequently expressed by AML blast cells (70-80% of cases) - where it can support myeloid differentiation, CD117 also has some value in establishing primitive differentiation as it is lost as cells mature