Auer rods are formed by fusion of the azurophilic granules and are less common in the M3v form of APL than they are in the hypergranular form. The finding of multiple Auer rods in a leukemic cell is common in AML, M3. Cytochemical staining of these leukemic cells in either variant of FAB, M3 AML would typically show intense myeloperoxidase and Sudan black B positivity in the cytoplasm. The nucleus somewhat resembles that of a monocyte, and this type of AML is sometimes confused with a monocytic or a myelomonocytic process. Staining for specific and nonspecific esterases is often helpful since promyelocytes show strong intracytoplasmic positivity with myeloperoxidase, Sudan black B and chloroacetate esterase and do not stain or are weakly positive with non-specific esterases.
Monocytic cells show intracytoplasmic positivity with the non-specific esterases.
Marked thrombocytopenia is the norm in AML, M3, whether the hypergranular or the microgranular variant. Note the presence of numerous Auer rods. The nucleus may be round or irregular with a folded or reniform appearance. The abnormal promyelocytes often contain numerous Auer rods. Individuals with the microgranular form more often present with a leukocytosis, as was the case in this patient.