The French-American-British cooperative group, in their seminal paper of 1976, described a promonocyte as follows: “This cell is similar to the monoblast but has a large nucleus with a cerebriform appearance; nucleoli may be present, but the cytoplasm is less basophilic, has a greyish ground-glass appearance and fine azurophilic granules are often scattered throughout.” They did not advise counting promonocytes with blast cells when distinguishing between acute leukemia and a myelodysplastic syndrome. In 2008, the WHO expert group described promonocytes as follows: “promonocytes have a delicately convoluted, folded or grooved nucleus with finely dispersed chromatin, a small indistinct or absent nucleolus, and finely granulated cytoplasm.” The two descriptions have some similarities and yet the WHO group advised that monoblasts and promonocytes be summated when estimating the blast count to establish a diagnosis of acute leukemia. They emphasised the importance of distinguishing promonocytes from “more mature but abnormal leukemic monocytes … which have more clumped chromatin … variably indented, folded nuclei and grey cytoplasm with more abundant lilac-coloured granules”.1
These images are from a 70-year-old woman who presented with septicemia and a purpuric rash, Her full blood count showed a white cell count of 36 × 109/l, a hemoglobin concentration of 89 g/l and a platelet count of 15 × 109/l. A diagnosis of acute monocytic leukemia was made, on the basis of morphology and cytochemical stains of peripheral blood and bone marrow. The left image compares a monoblast and a promonocyte. The monoblast (bottom) has a large nucleolus and a regular oval nucleus whereas the promonocyte (top) has a notch that leads into a groove crossing the nucleus and has smaller less distinct nucleoli and more obvious granules. The right image compares a promonocyte with an immature abnormal monocyte. The promonocyte (top) has a nucleolus and two superimposed nuclear lobes while the abnormal immature monocyte (bottom) is showing some definite chromatin condensation; avoiding classifying this latter cell as a promonocyte helps to avoid a misdiagnosis of chronic myelomonocytic leukemia as acute leukemia. It is the finely dispersed chromatin of the promonocyte that is the most important feature in distinguishing it from an immature monocyte.