When a spinal tap is performed correctly, the cell types found in the cytospin present a "snapshot" of the CSF cell content. When there is difficulty obtaining a clean tap, the cell types present will reflect the type of fluids that are co-mingled with the CSF. In traumatic taps, the cell types and proportions of red and white blood cells present will mirror that found in the patient's peripheral blood, as there has been a contamination of peripheral blood (from the trauma) mixed into the CSF during the sample collection.
In patients with softer bones (eg, infants and osteopenic patients), it is not uncommon for the contamination to come from bone marrow found inside the softened spinous process that was penetrated inadvertently during the procedure. When this occurs, any or all of cell types found within the bone marrow can be present in the cytospin.
In the image on this page, there are early nucleated RBC and WBC precursors, typical of contamination with bone marrow.