Medical Laboratory Scientist ASCP Practice Exam 2025 – Complete Study Guide

The classification of acute myeloid leukemia (AML) is based on the specific lineage of the leukemic cells and various cytochemical staining characteristics. In this context, the key feature is the positive reaction for alpha-naphthyl acetate esterase. This enzyme activity is a hallmark of monocytic differentiation.

When AML shows positive alpha-naphthyl acetate esterase activity, it indicates that the leukemic cells have a monocytic phenotype. This is characteristic of acute myeloid leukemia classification M4, which includes acute myelomonocytic leukemia. In this type, more than 20% of the cells in the bone marrow are of monocytic origin, and the positive esterase activity supports this monocytic differentiation.

Other classifications like M1 and M2 generally do not show a strong positive reaction for this enzyme since they are classified more for their myeloid precursors rather than monocytic features. M3 is associated with promyelocytic leukemia, and thus does not exhibit the monocytic characteristics that would lead to a positive result for alpha naphthyl acetate esterase. Therefore, the presence of this specific activity aligns well with the characteristics of M4 classification.