Bandemia is defined as the presence of immature neutrophil precursors exceeding 10 % of the total white blood cell (WBC) count. Also referred to as a “left shift,” the association between bandemia and the presence of infection was first identified in the late 1980s [1]. Since then, several studies have examined the diagnostic utility of bandemia, demonstrating predictive value for both the presence of infection [[2], [3], [4]], the identification of specific bacterial pathogens [3,5], and increased morbidity across various clinical settings [[6], [7], [8]]. As further illustration of the clear importance of bandemia in infectious etiologies, increased band counts above 10 % is included in the American College of Chest Physicians/Society of Critical Care Medicine's systemic inflammatory response syndrome (SIRS) criteria for defining the severity of sepsis and septic shock.

A study of nearly 1000 patients demonstrated that bandemia has high specificity for predicting bacteremia [4], while Cavallazzi et al. reported similarly high specificity in a cohort of 145 intensive care unit (ICU) patients for predicting infection [2]. Prasanna et al. demonstrated a strong correlation between bandemia trends and changes in Sequential Organ Failure Assessment (SOFA) scores among over 130 patients, suggesting that monitoring bandemia trends may be useful in predicting sepsis-related mortality [7]. Additional studies have established significant correlations between increased band proportion and in-hospital mortality in sepsis patients [3,6]. While these seminal studies established the role of bandemia as a diagnostic and prognostic tool, they largely focused on the presence of bandemia alongside with leukocytosis. However, in acute settings, typical markers of infection like elevated white blood cell (WBC) count and fever can be unreliable. Seigel et al. showed in a study of over 3000 patients that up to one-third of patients with culture-proven bacteremia are afebrile at presentation, and more than half of the patients have a normal level of WBC count [9].

When directly comparing the sensitivity of WBC count to bandemia for detecting infection, several studies found that bandemia was more sensitive, while WBC count was akin to “flipping a coin” [2,4,5]. Bandemia was shown to be independently predictive of bacteremia, even after controlling for WBC count [4]. These findings underscore the importance of bandemia as an independent marker of infection, especially in the absence of leukocytosis.

To date, no large-scale study has evaluated clinical outcomes in patients presenting with bandemia and a normal WBC count. In this study, we analyze a large international cohort of emergency department (ED) patients with isolated bandemia and normal WBC count and characterize their clinical outcomes.