What stain differentiates chronic myelogenous leukemia (CML) from a leukemoid reaction?

The differentiation of chronic myelogenous leukemia (CML) from a leukemoid reaction is effectively accomplished using leukocyte alkaline phosphatase (LAP) staining. In CML, there is typically a low LAP score, reflecting the neoplastic, immature granulocytes that are characteristic of the disease. This low activity is indicative of the presence of immature white blood cells that cannot produce adequate alkaline phosphatase due to their abnormal development.

Conversely, a leukemoid reaction, which is a benign response to infection or stress, is associated with a high LAP score. The mature neutrophils present in such reactions have active alkaline phosphatase, resulting in a more pronounced reaction.

Therefore, the leukocyte alkaline phosphatase stain serves as a crucial tool in the diagnostic distinction between these two conditions, highlighting the functional differences in leukocyte populations and their enzyme activity.