As noted above, a leukemoid reaction is typically a response to an underlying medical issue.

A severe neutrophilia with left shift is referred to as a leukemoid reaction.

The bone marrow in a leukemoid reaction, if examined, may be hypercellular but is otherwise typically unremarkable.

Causes of leukemoid reactions include:

In this setting, CML must be distinguished from a leukemoid reaction, which can have a similar appearance on a blood smear.

Basophils and eosinophils are almost universally increased; this feature may help differentiate CML from a leukemoid reaction.

Leukemoid reactions are generally benign and are not dangerous in and of themselves, although they are often a response to a significant disease state (see Causes below).

Conventionally, a leukocytosis exceeding 50,000 WBC/mm with a significant increase in early neutrophil precursors is referred to as a leukemoid reaction.

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 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.

Vesiculopustular eruption and leukemoid reaction in Down syndrome is a cutaneous condition, an extensive neonatal vesiculopustular eruption seen in patients with Down syndrome.

A leukocyte count above 25 to 30 x 10/L is termed a leukemoid reaction, which is the reaction of a healthy bone marrow to extreme stress, trauma, or infection.

The term leukemoid reaction describes an elevated white blood cell count, or leukocytosis, that is a physiological response to stress or infection (as opposed to a primary blood malignancy, such as leukemia).

The morphologic changes have also been described in myxedema associated with panhypopituitarism, vitamin B-12 and folate deficiency, multiple myeloma, enteroviral infections, malaria, muscular dystrophy, leukemoid reactions secondary to metastases to the bone marrow, and drug sensitivity, sulfa and valproate toxicities are examples.