The primary objective of this study was to find a low-cost, easily accessible and effective inflammatory marker that can predict ICU mortality, and to further evaluate its predictive power. To achieve this goal, we first divided the patients into the survival and death group to investigate the correlation between inflammatory markers of blood routine examination and in-hospital mortality in ICU patients, and it turned out that NLR had the best predictive ability. Then we determined the cutoff value of NLR and re-grouped the patients by NLR levels. We found that patients with lower or higher NLR levels were more likely to have higher mortality rate and longer ICU and hospital stay. Next, we incorporated NLR into the SAPS II and found that the addition of NLR can significantly improve the predictive power of SAPS II. Finally, we conducted a subgroup analysis based on ICU type, and the results were basically consistent with the overall population.
The predictive value of NLR has been widely studied, particularly in cardiovascular disease[11, 16], infectious disease[12, 17–19] and cancer[7–10, 20, 21]. Most previous studies suggested that the higher the NLR level, the worse the prognosis[7–11, 13, 16–18, 20, 21]; however, other studies suggest that low NLR level also impart poor prognosis[12, 19]. If we equally divided the patients into 3–5 groups based on NLR level, we can draw the conclusions that high NLR level indicate poor prognosis (Additional file 1: Table S1). But before analysis, we noted that patients with lower NLR level also seem to have poor prognosis. Thus we took a different grouping scheme and confirmed our conjecture by further analysis. This finding was in line with clinical experience and therefore easy to explain. Indeed, the prognosis is generally good when the clinical indicators are within the normal range, too high or too low are more likely associated with poor prognosis. The possible reason of elevated NLR leading to poor prognosis has been mentioned in many literatures, which mainly reflects enhanced systemic inflammation and stress response[7, 13, 22, 23]. However, the reason why low NLR levels are associated with poor prognosis remains unclear, and we speculated the following reasons for this. Decreased NLR is mainly due to decreased neutrophils. Neutrophils play a key role in the innate immune response, including directly killing pathogens by phagocytosis, releasing a variety of cytokines, activating T cells and so on [12, 24]. Therefore, a reduction in the circulating neutrophil count could lower the body’s response to microbial invasion. In addition, the reduced circulating neutrophil count could be ascribed to increased neutrophil adhesion to the vascular endothelium[25], which could also cause endothelial damage, leading to leukocyte aggregation and microvascular thrombosis[26]. Thus, the compromise of innate immunity and the increase in endothelial damage could collectively impair the prognosis of the patients[27].
Many previous studies have overlooked that low NLR levels can also lead to poor prognosis, this may be caused by the following reasons. 1) The number of patients with low NLR levels was small. There are only 580 patients with NLR ≤ 1, which was 2.66% of the total population. Together with the overall trend that higher NLR level is associated with worse prognosis, the small number of patients with low NLR and poor prognosis may have been neglected; 2) The main outcome indicators may have a certain influence on the conclusion. Previous studies have mostly focused on late-death (≥ 5 days) [8, 10, 13, 20, 22] and found that high NLR level could predict poor prognosis. But Riché et al. reported that low NLR level was associated with early death (< 5 days), while high NLR level was associated with late death[12]. Duggal et al have also suggested that increased NLR was a biomarker for increased length of stay in ICU patients [23]. Therefore, it is reasonable to draw conclusions that high NLR indicate high mortality from previous studies focusing on late death. However, in our study, around half of the in-hospital deaths (1512/3149, 48.02%) occurred within 5 days, so low NLR level may also lead to increased mortality could be noted in our study; 3) The study population may also have an impact on the conclusion. Multiple previous studies were conducted in patients with specific diseases, while our study focused on the universality of all ICU patients, so we included all ICU patients with no case selection. For patients in MICU, many diseases can present with lymphocytosis and neutropenia, including hematological malignancies such as acute lymphocytic leukemia and myelodysplastic syndrome [27–30]; hematopoietic system diseases like aplastic anemia[31]; rheumatic diseases like systemic lupus erythematosus[32]; and infectious etiology such as HIV,HBV and Epstein-Barr virus, etc. These patients are at elevated risk of bacterial and fungal infections, which accordingly have poor prognosis[33, 34]. This may be the reason for over-representation of the MICU patients with NLR ≤ 1 (3.36%). For postoperative patients in SICU, TSICU and CSRU, surgery will normally lead to elevated levels of NLR[35, 36]. On the one hand, tissue damage caused by trauma or surgery induced an acute inflammatory reaction, which leads to the accumulation of neutrophils[36, 37]; on the other hand, surgery and anesthesia exposed the body to a state of stress, which induces catecholamine and adrenocorticotropic hormone release, inducing the bone marrow, liver and spleen to produce neutrophils constantly and resulting in a massive recruitment of immature neutrophils into circulation[38]. In addition, cortisol inhibited the synthesis of lymphocyte nucleic acids, which leads to lymphopenia[39]. Therefore, postoperative patients should have a higher NLR. If the NLR is still at abnormally low levels for the postoperative patients, then the predominantly neutrophilic inflammatory response are probably not activated, but only lead to a transient type of lymphocytosis[40] and therefore cause poor prognosis. This is also consistent with previous reports that the mortality rate is significantly higher among trauma patients with lymphocytosis[41].
In this study, SAPS II was chosen as a tool for predicting the mortality. Although SAPS III had a better predictive ability, there were too many missing values due to the need for data within 1 hour after admission[42], so we chose to use SAPS II. Some studies had suggested that PLR also had the ability to predict mortality[10, 43, 44], therefore, we evaluated the predictive power of the PLR, and found that it does have prediction ability but not as good as NLR. When we continued to add PLR to the new SAPS II scoring model with NLR, the AUC value did not increase significantly, so we did not incorporate PLR into this model.
The major strengths of our study were the large sample size and including all ICU patients without selection bias. Furthermore, we also noticed that the mortality rate was elevated in patients with low NLR levels. Of course, there were some limitations of this study. First, this was a retrospective study, and therefore some important data might be missing. Some patients were excluded because of missing data on neutrophil or lymphocyte, and it was hard to explore the reasons for the missing data based on currently available information. Second, the conclusion of this study was qualitative, but not quantitative. We can only conclude that the addition of NLR can improve the performance of the SAPS II, but the NLR scores can’t be directly included into SAPS II and construct a new scoring model. However, we believed the results of the current study would be an important prompt to later scoring systems. Finally, although we conducted subgroup analysis in different types of ICU, in depth analyses were not undertaken because it’s not the aim of our study.