Leukocyte alkaline phosphatase (LAP) activity can be used to differentiate a leukemoid reaction from chronic myelogenous leukemia. 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 leukemoid leukocytosis show very high LAP score 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 are scored based on the amount of stain, the numbers are added up. This gives a potential LAP score of 0 to 400.
As shown in Table 1, LAP scores in a patient under normal circumstances would have a LAP score between 13 to 130. In a leukemoid reaction, scores greater than 160 would be expected. In cases of CML, the LAP score is low (less than 13) due to the fact that these cells are not being produced in response to an infection or stress but rather due to the neoplasm.
Table 1. Differentiating LAP score in Leukemoid Reaction versus CML.Condition | LAP Score |
Leukemoid Reaction | >160 |
Normal, healthy person | 13 to 130 |
CML | <13 |