Some cases of acute leukemia are composed entirely of undifferentiated-appearing blasts. These cases are difficult (or impossible) to diagnose morphologically (under the microscope) – you really need special tests like immunophenotyping in order to make a definitive diagnosis.

Other cases of acute leukemia have obvious morphologic clues. Acute promyelocytic leukemia (APL) falls into the latter category. It is composed of malignant promyelocytes, which often have a distinctive appearance.

But the really characteristic finding in APL is the faggot cell, so named because it contains a ton of Auer rods all piled up on each other, resembling a bundle of sticks (or faggot). When you see these, you can make the diagnosis of APL based on morphology alone, without waiting for molecular or cytogenetic studies (which will show the characteristic t(15;17) of APL – but which take some time to perform).

Making an immediate, morphologic diagnosis is critical in cases of APL, because these patients cannot be given routine acute myeloid leukemia chemotherapeutic agents. The malignant promyelocytes of APL contain lots of granules which have strong procoagulant activity. If you give the patient typical acute leukemia treatment, the promyelocytes will burst, the nasty granules will be released, and the patient will be at high risk for disseminated intravascular coagulation (DIC), a very dangerous syndrome in which patients bleed and clot all over the body.

Fortunately, there is a drug called all-trans retinoic acid (or ATRA) that works great for patients with APL because it causes the malignant promyelocytes to mature (into myelocytes, then metamyelocytes, then neutrophils). Then the patient can be treated with regular chemotherapy without risk of DIC.