Importance of distinguishing the promonocyte in leukemia

On rare occasions, a promonocyte may be difficult to morphologically distinguish from a promyelocyte, giving rise to the diagnosis of acute promyelocytic leukemia, particularly the microgranular variant. It is necessary to correctly diagnose these two types of leukemia, as treatment is different for each. Flow cytometry and cytogenetic/molecular studies are useful in distinguishing the two when morphology alone is equivocal. We report a case of acute monocytic leukemia in which the promonocytes morphologically masqueraded as promyelocytes.