The immunophenotyping and cytochemistry staining results given for this patient are included on the right to facilitate this discussion. The generalist laboratory professional does not often encounter flow cytometry or cytochemistry staining results but recalling some general trends will make these testing panels less intimidating. We will discuss cytochemistry staining first.
Interpreting cytochemistry staining:
- Myeloperoxidase (MPO) is an enzyme present in the granules of myeloid and monocytic cells. This enzyme is responsible for certain immunologic reactions that assist our innate/natural immune response. Cells of myeloid lineage (especially neutrophils) will stain positive for this enzyme, while cells of a lymphoid lineage will not. Acute myeloid leukemia (AML) is MPO positive, whereas ALL is MPO negative.
- Memory tool: Myeloperoxidase is positive in myeloid cells.
- Terminal deoxynucleotidyl transferase (TdT) is an enzyme present in the nucleus of a lymphocyte. This enzyme is involved in reactions related to DNA synthesis. It can sometimes be seen in the nucleus of a myeloid cell but is much stronger in the nuclei of lymphoid cells, such as those in ALL. As a result, ALL is TdT positive.
Interpreting immunophenotyping:
- Immunophenotyping using monoclonal antibodies is utilized to identify cluster of differentiation (CD) markers on the lymphoblast cell surface.
- CD2, 3, 5, and 7 are present on T cell blasts
- Memory tool: The CD markers that are tinier (less) than ten are associated with T cell blasts.
- CD10, 19, 20, 22, and 34 are present on B cell blasts
- Memory tool: The CD markers larger than ten are associated with B cell blasts UNLESS they are CD13, 15, or 33 (see next bullet).
- CD13, 15, and 33 are on myeloid blasts, NOT lymphoid
