Review articleAn update on the etiology and diagnostic evaluation of a leukemoid reaction
Introduction
A leukemoid reaction (LR) is a hematological disorder, defined by a leukocyte count greater than 50,000 cells/μL, caused by reactive causes outside the bone marrow [1], [2]. It is characterized by a significant increase in mature neutrophils in the peripheral blood and a differential count showing marked left shift [2]. The diagnosis of LR is based on the exclusion of chronic myelogenous leukemia (CML) and chronic neutrophilic leukemia (CNL). The absence of immature cells, basophilia or monocytosis, increased leukocyte alkaline phosphatase (LAP), and the absence of the bcr/abl translocation distinguishes LR from CML. CNL is a rare, distinct myeloproliferative syndrome with a poor prognosis [3]. The differential diagnosis between LR and CNL may be difficult or even impossible because both conditions share identical morphological features, including a raised LAP score and the absence of the bcr/abl translocation [3].
The current review discusses the different causes of LR, pointing out the difficulties in the differential diagnosis. Moreover, a diagnostic approach to a rational clinical and laboratory evaluation that mainly refers to an isolated LR is proposed.
Section snippets
Etiology
The major causes of LRs are severe infections, intoxications, malignancies, severe hemorrhage, or acute hemolysis (Table 1). A variety of infections, such as colitis due to Clostridium difficile, disseminated tuberculosis [4], and severe shigellosis, have been associated with an LR. C. difficile colitis with an LR appears to be associated with a much higher mortality rate (∼ 50%) [5]. A study of 111 children with Shigella dysenteriae type 1 infection revealed 25 patients with LR. Its presence
Diagnostic evaluation
An LR is, by definition, diagnosed after the exclusion of a malignant hematological disorder. In the case of concomitant polycythemia vera or thrombocytosis,a diagnostic approach concerning general myeloproliferative diseases should be made. However, the underlying cause of an LR is usually obvious. The clinician should obtain a clear history which, together with a good physical examination and limited imaging procedures, may provide significant clues to a diagnosis, such as a neoplasm or an
Conclusions
A LR is a rare condition that can be challenging and that may require a careful diagnostic work-up. The diagnosis is verified by a combination of the following: (i) a complete blood count with a peripheral blood smear that shows marked mature neutrophilia with a left shift; (ii) a high leukocyte alkaline phosphatase (LAP) score; (iii) hypercellular bone marrow with intact maturation and morphology of all the elements; (iv) the absence of cytogenetic abnormalities by cytogenetic-molecular
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