Acute monocytic leukemia with chloroacetate esterase positivity. FAB M4 or M5?

French-American-British criteria for the diagnosis of acute monocytic leukemia (M5) require that 80% of nonerythroid bone marrow cells consist of monoblasts, promonocytes, and/or monocytes. Monocytic differentiation is demonstrated by fluoride-sensitive nonspecific esterase positivity. Chloroacetate esterase positivity is accepted as a marker of granulocytic differentiation. Three cases fulfilling French-American-British criteria for M5 showed fluoride-sensitive nonspecific esterase positivity in up to 100% of nonerythroid marrow cells but also exhibited strong chloroacetate esterase positivity in 20% to 90% of the same population. Less than 5% of blasts stained for Sudan black B and peroxidase. These cases may be viewed as chloroacetic esterase-positive acute monocytic leukemia or as acute myelomonocytic leukemia. The authors favor the former because the cases were myeloperoxidase negative; however, these cases indicate that chloroacetate esterase may not be a specific marker for granulocytic differentiation.