Q. In which type of AML do you see the most Auer rods?

A. Auer rods are elongated structures seen in malignant cells of the neutrophil lineage. Mostly, they are seen in myeloblasts – but you can see them in any stage of maturation (even in mature neutrophils). They are really just linear groupings of primary granules (therefore, you only see Auer rods in neutrophilic cells – not monocytes, or red cells, or any other type of cell). When you see an Auer rod, you know two things:

1. The population of cells you’re looking at is malignant
2. The malignancy is one that involves the neutrophil series.

Usually, seeing an Auer rod means that you’re looking at a case of acute myeloid leukemia (AML). In fact, for many years, Auer rods were considered virtually diagnostic of AML. Over the last 30 years or so, though, it’s become pretty clear that you can also see Auer rods in disorders other than AML (specifically, you can see them in a rare myeloid malignancy called chronic myelomonocytic leukemia, and in a specific type of myelodysplastic syndrome called refractory anemia with excess blasts-2 (RAEB-2)). As you’re starting out in pathology, though, it’s enough to just remember that Auer rods are seen in AML.

Auer rods are seen in several (but not all!) types of AML. Using the older FAB classification, the types of AML you see Auer rods in are AML-M1 (acute myeloblastic leukemia without maturation), AML-M2 (acute myeloblastic leukemia with maturation), AML-M3 (acute promyelocytic leukemia), AML-M4 (acute myelomonocytic leukemia), AML-M5 (acute monoblastic and monocytic leukemia), and AML-M6 (acute erythroblastic leukemia). The frequency with which you see Auer rods in these types of AML differs though.

This type of AML is composed of just myeloblasts (with no maturation into more mature cells, like promyelocytes). In some cases, you’ll see definitive Auer rods, but in other cases you’ll be hard-pressed to find an Auer rod.

This type of AML is characterized by myeloblasts and maturing neutrophilic cells. Auer rods are frequently present.

In this type of AML, you see mostly promyelocytes. There is a strange and characteristic cell in AML-M3 called the faggot cell (faggot meaning bundle of sticks). This cell has tons of  Auer rods in it (like 30 or 40!). It’s pathognomonic for this type of AML. Single Auer rods are pretty uncommon.

This is a dual-lineage AML that involves both the monocytic and neutrophilic series. You’ll see a lot of monocytic precursors (which won’t have Auer rods) but also a lot of neutrophilic precursors (which may have Auer rods). Overall, you don’t see a ton of Auer rods, because there are not as many myeloblasts as there are in other types of AML (such as M1 or M2).

Wait, isn’t this an AML composed of monocytic precursors? Yes, it is – but there can be a few myeloblasts lurking about too. Therefore, you might see an Auer rod here or there (in the myeloblasts, not in the monocytic cells). They won’t be numerous, though.

Same idea here: this is an AML composed mostly of non-neutrophilic-lineage cells (in this case, erythroblasts). However, there are some cases of M6 that have a few myeloblasts around too. In these cases, you might see an Auer rod here or there (in the myeloblasts) – but they won’t be too numerous, because this leukemia is composed predominantly of erythroid precursors.

So to sum up: Auer rods are most frequent in AML-M2. You also see a fair number of them in AML-M1, AML-M3 (in faggot cells), and AML-M4. They are rarely seen in AML-M5 and AML-M6 (in cases that have a few myeloblasts around).