The leukocyte alkaine phosphatase (LAP) score is often used in patients with an elevated WBC to differentiate a reactive process from chronic myelogenous leukemia.The score in the latter is low while it is within the normal reference range or elevated in the former condition. Naphthyl AS-B1 phosphate is hydrolyzed to phosphate and an aryl maphthylthalamide by alkalline phosphatase in the cytoplasm of WBCs. The aryl naphthylthalamide is coupled to diazonium salt (fast red violet LB) forming an insoluble blue dye within the cytolasm of the WBCs. Neutrophils and bands are scored (0 to 4+) and the total score of 100 of these cells is the LAP score. Variable intensity of blue staining can be seen in the leukocytes in this peripheral smear from a patient with a reactive leukocytosis.