ObjectiveTo explore the predictive value of systemic immune-inflammation index (SII) combined with neutrophil-monocyte ratio (NMR) on postoperative anastomotic leakage in elderly colon cancer. MethodsThe clinical data of 493 elderly colon cancer patients who attended the Department of General Surgery of the First Hospital of Lanzhou University from January 2018 to October 2023 were retrospectively analysed, and divided into an anastomotic leakage group (n=29) and a non-anastomotic leakage group (n=464) according to the occurrence of anastomotic leakage or not, and the differences between the two groups in terms of SII and NMR at different time points were compared. Area under the curve (AUC) of receiver operating characteristic (ROC) was used to compare the predictive value of SII, NMR and the combination of the two on the occurrence of anastomotic leakage after surgery in elderly colon cancer patients. Logistic regression was used to analyse the independent risk factors for postoperative anastomotic leakage in elderly colon cancer patients. ResultsThe SII and NMR in the anastomotic leakage group were higher than those in the non-anastomotic leakage group on the 3 rd and 5 th day after operation (P<0.05). ROC curve analysis showed that the AUC values for SII on postoperative day 3, NMR on postoperative day 5, and the combination of the two to predict anastomotic leakage were 0.613, 0.743, and 0.750, respectively. The results of DeLong’s test suggested that the difference between the AUC values of NMR on postoperative day 5 combined with SII on postoperative day 3 and SII on postoperative day 3 was statistically significant (P=0.047). Multifactorial logistic regression analysis showed that age, male, diabetes, preoperative radiotherapy and chemotherapy, tumor located in the left colon, SII on postoperative day 3, and NMR on postoperative day 5 were independent risk factors for postoperative anastomotic leakage in elderly patients with colorectal cancer (all P<0.05). ConclusionsPostoperative day 3 SII combined with postoperative day 5 NMR, postoperative day 3 SII, and postoperative day 5 NMR all have predictive value for postoperative anastomotic leakage in elderly patients with colorectal cancer. Both have the potential to serve as important predictors of postoperative anastomotic leakage in elderly patients with colon cancer.