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A diagnosis of acute monocytic leukemia(AML-M5) based on α-naphthyl butyrateesterase (α-NB) staining has some problems,because AML-M5 leukemic cells often showweak or faint positivity on α-NB staining. Inthese situations, some cases of AML-M5tend to be misdiagnosed as AML-M0. Therefore, we evaluated the significance ofweak or faint α-NB staining in AML-M5diagnosed by flow cytometry (FCM). Nineteen AML cases in which leukemic cellswere negative for naphthol AS-D chloroac-etate esterase staining were studied. ForFCM, we defined leukemic cells as having amonocytic nature when more than 10% ofthe leukemic cells were positive for at leastone of the following antigens: CD4, CD11c,CD14, and CD64. The monocytic naturedetermined by FCM was consistent with pos-itive or weak positivity on α-NB staining. Five of 6 cases in which leukemic cellsexhibited faint positivity for α-NB stainingcould be diagnosed as AML-M5 by FCM,while negative α-NB staining was consistentwith a diagnosis of AML-M0. These resultssuggest that AML-M5 should be taken intoconsideration even when leukemic cells arefaintly positive for α-NB staining.
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