Leukemoid reaction — Leukocytosis exceeding 50,000/µL is referred to as a leukemoid reaction and is characterized by a significant increase in early neutrophil precursors in the peripheral blood. The differential count has a marked "left shift," evidenced by the presence of myelocytes and metamyelocytes, and increased numbers of band forms in the peripheral blood. Promyelocytes and myeloblasts may occasionally be observed in severe reactions. Proliferation of all the normal myeloid elements is observed in the bone marrow in leukemoid reactions, in contrast to acute leukemia, in which the immature elements predominate.
The most important hematologic disease when evaluating a patient with a leukemoid reaction is to rule out chronic myelogenous leukemia.
LAP level is high, or normal in leukemoid reaction. In CML, LAP level is low.