Which test is MOST helpful in establishing a diagnosis in a patient with elevated WBC and toxic granulation?

The Leukocyte alkaline phosphatase stain (LAP stain) is the most helpful test in establishing a diagnosis for a patient with elevated white blood cell (WBC) counts and toxic granulation. Toxic granulation is often associated with reactive leukocytosis, commonly seen in infections, inflammation, or stress responses in the body. The LAP stain specifically evaluates the activity of alkaline phosphatase in leukocytes, and elevated LAP scores can indicate a reactive process.

In the context of the question, an elevated LAP score would support a diagnosis of conditions such as leukemoid reactions or myeloproliferative disorders while also distinguishing these from chronic myelogenous leukemia (CML), where the LAP score is typically low. Therefore, the LAP stain provides crucial information not only in confirming the presence of leukocyte activity but also in guiding the clinical interpretation of the elevated WBC count and concurrent toxic granulation.

The other tests listed are useful in different contexts but do not provide the same level of specificity for interpreting elevated WBC counts with toxic granulation.