Which stain is used to differentiate chronic myelogenous leukemia from a leukemoid reaction?

The leukocyte alkaline phosphatase (LAP) stain is the most appropriate choice for differentiating chronic myelogenous leukemia (CML) from a leukemoid reaction. In CML, the LAP score tends to be low due to the presence of immature granulocytes and a predominance of myeloblasts, which do not express alkaline phosphatase as intensively. In contrast, a leukemoid reaction, which is often a response to infection or stress, typically shows a high LAP score due to the elevated levels of mature neutrophils.

This differentiation is crucial because both conditions can present with elevated white blood cell counts, but their underlying causes and subsequent management differ significantly. Understanding LAP activity helps clinicians make more informed diagnoses and treatment decisions. Other stains like Giemsa, Wright's, and Prussian blue serve different purposes in hematology and do not provide the necessary insights for this specific differentiation.