This image shows a CSF cytospin prepared from a known leukemia patient who presented during therapy with new onset of severe headache and mental status changes. The cell count revealed 350 white blood cells (WBCs) and 5 red blood cells (RBCs)/µL.
Notice the large mononuclear cells that are the predominant population in this sample. They are quite large compared to the few normal lymphocytes and occasional RBC that are present. Notice the very fine chromatin and markedly irregular nuclear shape of the blasts. These cells should not be confused with monocytes. The cytoplasm is lymphoid, without the fine, "ground glass" cytoplasm that is typical for a monocyte.
Another differentiating feature is the scant cytoplasm, which discourages identification as a monocyte. Although monocytes can have irregular nuclear shapes, these cells have nuclear irregularities that exceed those seen in normal monocytes.