Polynuclear neutrophils can play dual roles in sepsis: on the one hand they mediate major antimicrobial activities and on the other hand they can contribute to the development of multiple organ failure [1]. Nonetheless, in spite of the importance of these cells in sepsis, the influence of the circulating neutrophil count (CNC) on the prognosis of septic patients with this pathology has not been properly evaluated.

We analyzed the association between CNC and outcome in two cohorts of patients with diagnostic criteria of septic shock (SS) [2]: the first was recruited in the context of a single center study (EXPRESS study, discovery cohort, n = 195; Table 1), and the second in the context of a multi-centric study (GRECIA study, validation cohort, n = 194; Table 2). Written informed consent was obtained from each patient or their legal representative. The two studies were approved by the Research Ethics Committee of the Hospital Clínico Universitario, Valladolid, Spain (for the EXPRESS study) and Hospital Universitario Río Hortega, Valladolid, Spain (coordinating center for the GRECIA study).

Table 1 Clinical characteristics of the patients in the discovery study in survivors and non-survivors at 28 days
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Table 2 Clinical characteristics of the patients in the validation study in survivors and non-survivors at 28 days
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When patients of the discovery cohort were split based on deciles for CNC at SS diagnosis, those with CNC <7,226 cells/mm3 (decile 2) died earlier than the other non-survivors (Figure 1). Multivariate Cox regression analysis showed that patients with CNC below this cutoff value had an almost two-fold risk of death (Figure 1). The cutoff value was evaluated again in the validation cohort, with similar results (Figure 1). Counts of other leukocyte subtypes had no significant association with outcome.

Figure 1
figure 1

Impact of circulating neutrophil count on mortality: Kaplan-Meier survival curves. Groups were compared by the log-rank test (Mantel- Haenzel). Bottom: multivariate Cox regression analysis for mortality risk. Circulating neutrophil count (CNC) was adjusted by age, sex and Acute Physiology and Chronic Health Evaluation II score. Time was censored at 28 days following diagnosis. CI, confidence interval; Cum, cumulative; HR, hazard ratio.

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Although normal reference values in blood vary depending on sex, race and age, available literature supports that 7,226 cells/mm3 is at the upper limit of normal CNC values [3]. Patients with insufficient numbers of circulating neutrophils during the early stages of SS could have difficulties mounting effective innate responses against the invading microbe(s). Increased neutrophil adhesion to the vascular endothelium in sepsis could contribute to lower CNC. Neutrophils adhered to the blood vessel wall seem to induce endothelial damage [4], forming leukocyte aggregates that could lead to microvascular thrombosis [1, 5]. Host immunity compromise and/or increased endothelial damage could both impair outcome in these patients.

CNC at diagnosis is a major prognostic factor in SS. Our work provides a CNC cutoff that is potentially useful as a prognostic indicator.