Cells are characterized by both granulocytic and monocytic
differentiation (color plate 157 and 158).
On Romanowsky stained smear it is usually easy to find
cells with primary granules (granulocytic differentiation)
as well as cells with folded nuclei and moderate to abundant
cytoplasm (monocytic differentiation).
Peroxidase, sudan black, and NSE positive in 20 to 80%
of the cells.
When morphologic and cytochemical similarities make the
distinction between A$ and A@ difficult, the diagnosis of
M4 can be supported by finding a serum lysozyme level exceeding
three times the normal level and a peripheral blood monocyte
count of greater than 5X10(9)/liter.