[2] The peripheral blood smear may show myelocytes, metamyelocytes, promyelocytes, and rarely myeloblasts; however, there is a mixture of early mature neutrophil precursors, in contrast to the immature forms typically seen in acute leukemia.

Serum leukocyte alkaline phosphatase is normal or elevated in leukemoid reaction, but is depressed in chronic myelogenous leukemia.

However, leukemoid reactions can resemble more serious conditions such as chronic myelogenous leukemia (CML), which can present with identical findings on the peripheral blood smear.

Historically, various clues including the leukocyte alkaline phosphatase score and the presence of basophilia were used to distinguish CML from a leukemoid reaction.

However, at present the test of choice in adults to distinguish CML is an assay for the presence of the Philadelphia chromosome, either via cytogenetics and FISH, or via PCR for the BCR/ABL fusion gene.