A leukemoid reaction is a benign condition that may present similarly to CML on the complete blood count and peripheral blood smear. Although it is not performed often due to cytogenetic and molecular testing, leukocyte alkaline phosphatase (LAP) activity can be used to differentiate a leukemoid reaction from CML. It is determined by using a cytochemical stain to detect alkaline phosphatase activity in the neutrophils and bands. Normal neutrophils and bands show cytoplasmic alkaline phosphatase activity in the form of dark staining. Patients with a leukemoid reaction show very high LAP scores due to high cytoplasmic alkaline phosphatase activity. On the other hand, in patients with CML, the neutrophils show a very weak reaction to the alkaline phosphatase staining (low LAP score).
The LAP score is derived by counting one hundred neutrophils (and/or bands) and assessing the amount of stain retained in each of these neutrophilic cells. Each cell is assigned a score between 0 (no stain retained) to 4 (very dark staining due to the granules retaining the stain). Once each cell of the 100 neutrophils is scored based on the amount of stain, the numbers are added up. This gives a potential LAP score of 0 to 400.
