Blast with Auer rods

Some types of acute leukemia are composed of only blasts (no differentiating neutrophils, no monocytic precursors, just a sea of blasts). In those cases, look for Auer rods. A blast with an Auer rod can only be a myeloblast! It cannot be a lymphoblast, or a monoblast, or any other kind of blast. So if you see blasts with Auer rods, you know it is some type of acute myeloid leukemia. Remember, though, that the converse is not true: just because you don’t see Auer rods, that does not mean that the blast is not a myeloblast. Some myeloblasts have Auer rods, and some don’t. So if you see Auer rods, it is an AML. If you don’t, it still could be an AML.