Leukemoid reaction

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

Definition and diagnosis
Conventionally, a leukocytosis exceeding 50,000 WBC/mm3 with a significant increase in early neutrophil precursors is referred to as a leukemoid reaction. The peripheral blood smear may show myelocytes, metamyelocytes, promyelocytes, and even myeloblasts; however, there is a mix of early mature neutrophil precursors, in contrast to the immature forms typically seen in acute leukemia. The bone marrow in a leukemoid reaction, if examined, may be hypercellular but is otherwise typically unremarkable.

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). 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. 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 protein. The LAP (Leukocyte Alkaline Phosphatase) score is high in reactive states but is low in CML. In cases where the diagnosis is uncertain, a qualified hematologist or oncologist should be consulted.

Causes of leukemoid reaction
As noted above, a leukemoid reaction is typically a response to an underlying medical issue. Causes of leukemoid reactions include:
 * Hemorrhage
 * Drugs
 * Use of glucocorticoids
 * Use of G-CSF or related growth factors
 * All-trans retinoic acid (ATRA)
 * Infections (e.g. tuberculosis, pertussis)
 * As a feature of Trisomy 21 in infancy (incidence of ~10%)
 * As a paraneoplastic phenomenon (rare)