In addition to diagnosing and subclassifying AML, flow cytometry is needed to recognize the entities of undifferentiated acute leukemia, bilineage leukemia, and biphenotypic leukemia.10 Lineage-specific antigens include myeloperoxidase (myeloid), cytoplasmic CD3 (T-lymphoblasts), cytoplasmic CD22 (B-lym-phoblasts), cytoplasmic IgM (B-lymphoblasts) and cytoplasmic CD79a (B-cell lymphoblasts).12 In undifferentiated acute leukemias, no lineage-specific antigen is detected,10 and often only one lineage-associated antigen (such as CD13 or CD33) is positive by flow.10 Non-lineage specific primitive stem cell markers, such as CD34, CD38, and HLA-DR, may be present.90
Bilineage leukemia occurs when blasts derive from two distinct lineages.10 These leukemias are sometimes associated with the Philadelphia chromosome, t(9;22), or the MLL gene, 11q23.90
In biphenotypic leukemias, blasts are positive for two or three distinct lineages on the same cell.10 Specific criteria are used to make this diagnosis. One commonly used scoring system is the Royal Marsden criteria.12100 Blasts with greater than two points for two or more lineages are considered biphenotypic,12 and are considered distinct from AML with aberrant coexpression of lymphoid markers.88 CD7 is the most frequent lymphoid-associated antigen expressed in such cases.88
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