The usefulness of CD64, other monocyte-associated antigens, and CD45 gating in the subclassification of acute myeloid leukemias with monocytic differentiation.

Immunophenotyping by flow cytometry is widely used in the diagnosis and subclassification of acute myeloid leukemia (AML). CD14 is the monocyte-associated antigen most widely used to identify AML with monocytic differentiation (French-American-British classes M4 and M5); however, we observed that CD14 expression is frequently diminished or absent in such cases. To identify monocyte-associated antigens that might improve recognition of AML M4 and M5, we used 3-color flow cytometry and a panel of antibodies reported to distinguish cells of monocytic lineage in 44 cases of AML. In addition, CD45 vs logarithmic side scatter plots were analyzed in all cases. As expected, CD14 was highly specific but was only moderately sensitive for monocytic differentiation. CD64 had the best-combined sensitivity and specificity for AML M4 and M5. CD45 vs logarithmic side scatter analysis showed a higher percentage of monocytes in AML M4 and M5 compared with nonmonocytic AML. CD64 was expressed in 5 of 5 cases of acute promyelocytic leukemia (AML M3), but the intensity of staining was significantly less in AML M3 than in AML M4 and M5. Our findings show that addition of CD64 and CD45 vs logarithmic side scatter analysis to CD14 greatly improves flow cytometric detection of AML with monocytic differentiation and that CD64, also expressed in AML M3, may help distinguish AML M3 from other subtypes.