Although extremely high WBC count in the absence of myeloproliferative disease may be associated with solid tumors and other causes, infections are often the most common cause of leukemoid reaction (LR), including tuberculosis, Clostridiodes difficile colitis, shigellosis, salmonellosis, pneumonia, abscesses, as well as parasitic infections (eg, malaria), fungal infections (mucormycosis), and viral diseases (eg, HIV, EBV, Chickungunya fever).1-4
In a study of 173 hospitalized patients (mean age 69 y) with leukemoid reaction (defined in this study as WBC ≥30,000/µl), infection was the most common cause of LR (48%), followed by tissue ischemia/stress (28%), inflammation (eg, pancreatitis, diverticulitis without perforation) and obstetric diagnoses (7% each) and malignant tumor (5%).1
In the same study, the most common infections were “sepsis”, pneumonia and urinary tract infections. Bacteremia was documented in 13%, while Clostridiodes difficile toxin assay was positive in 7% of patients. The highest WBC counts were observed in patients with either a positive blood culture or positive C. difficile toxin. In-hospital mortality rate was very high at 62%.
Similarly, in a study involving 105 hospitalized patients, the most common cause was infection, followed by malignancy and other causes. 2 In a smaller study of 25 patients with “extreme” leukocytosis (defined as WBC ≥50,000/µl) infection was considered the cause in 52% and malignancy in 44% of patients; about one-third were bacteremic (eg, Pseudomonas sp, Streptococcus pneumoniae, E. coli).3
Bonus Pearl: Did you know that besides infections and malignancy, drugs (eg, corticosteroids, epinephrine) and ingestion of ethylene glycol have also been associated with LR? 1,3,4
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- Potasman I, Grupper M. Leukemoid reaction:Spectrum and prognosis of 173 adult patients. Clin Infect Dis 2013;57:e177-81. https://pubmed.ncbi.nlm.nih.gov/23994818/
- Portich JP, Faulhaber GAM. Leudemoid reaction: A 21st-century study. https://pubmed.ncbi.nlm.nih.gov/31765058/
- Halkes CJM, Dijstelbloem HM, Eelman Rooda SJ, et al. Extreme leucocytosis: not always leukaemia. The Netherlands J Med 2007;65:248-51. https://pubmed.ncbi.nlm.nih.gov/17656811/
- Kumar P, Charaniya R, Sahoo R, et al. Leukemoid reaction in Chickungunya fever. J Clin Diagn Res 2016;10:OD05-OD06. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4948452/
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