In most clinical hematology laboratories, an initial blood count is performed by an automated cell counting instrument. Additionally, most of these instruments also produce a five-part differential count, indicating the percentage of neutrophils, lymphocytes, monocytes, basophils, and eosinophils. Some instruments can also provide information about cellular immaturity and abnormal cellular morphologies.
Occasionally, atypical cells, similar to those shown in the image to the right, would be flagged, at which point a smear review would be required to make an identification. In cases where there are automated instrument differential flags or there are other indications that atypical cells may be present, a review of the smear is indicated.