In our study, the NLR measured on admission, was associated with the development of postoperative AKI in patients who underwent non-cardiac surgery under general anaesthesia. The cut-off value of the NLR was 3.55, the sensitivity of identifying AKI was 86.4 %,and the specificity was 51.9 %. Preoperative high NLR was an independent risk factor associated with AKI after non-cardiac surgery (OR, 2.410; 95 % CI: 1.371–4.335).
Inflammation plays an important role in the development of AKI . Some inflammatory marker can predict AKI, such as IL-18 , IL-10 , IL-6  and α-1 microglobulin . However, these indicators are not carried out as routine examinations in various hospitals; on the other hand, some indicators are only measured postoperatively related to postoperative AKI, which cannot achieve an early warning, so as to carry out intervention in the perioperative period.
The NLR which can be calculated using data from preoperative blood routine test, is a reliable marker for the systemic inflammatory response. Because the number of neutrophils represents the body's inflammatory response, and the number of lymphocytes represents the body's response to oxidative stress . In the inflammatory response, lymphocytes can be apoptotic, while neutrophils proliferate. Therefore, to a certain extent, the NLR indicates the balance of the inflammatory and anti-inflammatory reactions. A large number of studies have reported that the high NLR is closely related to the development and prognosis of various diseases, such as coronary artery disease, cancer and other diseases [25–27]. In the field of nephorology, Bu et al. reported that NLR value > 17.11 was correlated with septic AKI. Another retrospective study reported by Kim et al. indicated that preoperative NLR value > 11.7 was associated with early postoperative AKI in burn-injured patients. These two groups of patients are at high risk for postoperative AKI, however, our study included a wider range of patients. By using the optimal cut-off value obtained in our study, the rate of missed diagnosis is lower, which enables clinicians to be more alert to the occurrence of postoperative AKI in patients, strengthen preoperative prevention and management (e.g., avoid the use of nephrotoxic drugs or angiography) and postoperative renal function problems.
The association of preoperative high NLR with postoperative AKI may be the result of neutrophil activation prior to surgery. This, may further lead to endothelial injury and activation of coagulation pathway after surgery, which can stimulate the body to produce inflammatory mediators, induce systemic inflammatory response, and lead to postoperative AKI. Our study focused on the effect of preoperative basic state of patients on postoperative AKI, but also included intraoperative patient conditions, such as hemodynamic issues. There were significant differences in many indicators between the high and low NLR groups, and the factor of P < 0.1 was included to correct these indicators. The method of LASSO regression, which is a compression estimation method based on reducing variable set, is adopted to obtain a concise and effective model. A multi-factor regression model (AUC = 0.817) was constructed after excluding the interference of age, hypertension, surgical type and other factors. Moreover, through the likelihood ratio validation, it was proved again that preoperative high NLR was an independent risk factor for AKI after non-cardiac surgery, and that preoperative high NLR may have important value in predicting the occurrence of AKI after non-cardiac surgery.