Leukemoid reaction versus chronic myeloid leukemia (CML): which features are characteristic of a leukemoid reaction?

• A. WBC >50,000 with high LAP and more mature neutrophils
• B. WBC >100,000 with BCR-ABL fusion
• C. Low LAP score with immature neutrophils
• D. WBC around 3,000 with lymphocytosis

Answer: (A)

Leukemoid reaction is a reactive neutrophilic leukocytosis driven by infection or inflammation, so the neutrophils are functionally active and LAP is elevated. The hallmark is a very high WBC count with predominantly mature neutrophils and a pronounced left shift, along with a high leukocyte alkaline phosphatase score. This pattern helps distinguish it from chronic myeloid leukemia, where the neutrophils are part of a malignant clone with a low LAP score and often many immature forms; molecular testing would reveal the BCR-ABL fusion in CML. So, the combination of markedly increased WBC, high LAP, and mostly mature neutrophils is characteristic of a leukemoid reaction.