Leukemoid reaction: A 21st‐century cohort study

Leukemoid reaction (leukocyte count >50 cells ×10 9  L) is a rare but extremely relevant finding. Since little has been published on this condition's clinical relevance and prognosis, we investigated leukemoid reaction in patients with a white blood cell count of >50 × 10 9  L, including etiology and outcomes. Methods This retrospective cohort study included all patients at a Brazilian tertiary hospital between January 2016 and July 2018 > 18 years with a total leukocyte count >50 cells×10 9  L. Demographics, complete blood count, clinical features, and the exams used to diagnose and determine leukemoid reaction etiology were analyzed. A Kaplan‐Meyer survival analysis was performed, and a binary logistic regression model identified variables associated with death. Results Of the 267 cases with white blood cell count of >50 × 10 9 , 162/267 (60%) were secondary to hematopoietic neoplasm and 105/267 (40%) presenting as a true leukemoid reaction. The primary causes of the true leukemoid reaction cases were infection (59), nonhematopoietic neoplasm (17), or other causes (29). Patient deaths (66) differed significantly between groups ( P  < .001, log‐rank [Mantel‐Cox] Test). Lower hemoglobin, older age, and increased segmented neutrophil count were associated with increased risk of death. Conclusions This was a modern cohort analysis of leukemoid reactions, inclusive of all etiologies. The most common cause was infection, which involved several microorganisms. Paraneoplastic leukemoid reaction was also common. Both conditions have a poor prognosis with high mortality, being a major medical challenge.