The page below is a sample from the LabCE course White Cell and Platelet Disorders: Peripheral Blood Clues to Nonneoplastic Conditions. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Leukemoid Reaction

The term "leukemoid reaction" is used to describe a condition where peripheral white blood cells on a stained blood smear may have some resemblance to leukemia cells. Quantatively, in a leukemoid reaction, the neutrophil count may be as high as 50.0 X 109/L with more immature cells, particularly myelocytes, than are usually present in toxic left-shift syndromes.
The presence of immature cells in a leukemoid reaction awakens thoughts of leukemia. Great care must be taken to make a distinct differentiation between aberrant white blood cell proliferations (possible leukemia) and a benign but exaggerated granulocytic proliferative response (leukemoid reaction). Flow cytometry results, molecular or cytogenetic testing, and the leukocyte alkaline phosphatase (LAP) score can be helpful in differentiating these two conditions.
This particular peripheral smear represents a leukemoid reaction.