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Background The impact of preoperative anemia on a survival outcome and the importance of correcting preoperative anemia in patients with colorectal cancer (CRC) remain controversial. This study aimed to explore how preoperative anemia affects the long-term survival of patients undergoing colorectal cancer surgery. Methods This was a retrospective cohort study in which adult patients underwent surgical resection for colorectal cancer between January 1, 2008, and December 31, 2014, at a large tertiary cancer center. A total of 7436 patients were enrolled in this study. Anemia was defined according to the diagnostic criteria of China (hemoglobin level < 110 g/L for women and < 120 g/L for men). The median follow-up time was 120.5 months (10.0 years). Inverse probability of treatment weighting (IPTW) using the propensity score was used to reduce selection bias. Overall survival (OS) and disease-free survival (DFS) were compared between patients with and without preoperative anemia using the Kaplan–Meier estimator and the weighted log-rank test based on IPTW. Univariate and multivariate Cox proportional hazards models were used to assess factors associated with OS and DFS. Multivariable Cox regression was also used to assess red blood cell (RBC) transfusion associations between preoperative anemia and outcomes. Results After IPTW adjustment, clinical profiles were similar, except that tumor location and TNM stage remained imbalanced between the preoperative anemia and preoperative non-anemia groups (p < 0.001). IPTW analysis showed that the 5-year OS rate (71.3 vs. 78.6%, p < 0.001) and the 5-year DFS rate (63.9 vs. 70.9%, p < 0.001) were significantly lower in the preoperative anemia group. Multivariate analysis showed that preoperative anemia was associated with poorer OS and DFS, while RBC transfusion may improve OS (hazard ratio [HR] 0.54, p = 0.054) and DFS (HR 0.50, p = 0.020) in CRC patients with preoperative anemia. Conclusions Preoperative anemia is an independent risk factor for survival in patients undergoing colorectal surgery. Strategies to reduce preoperative anemia in patients with CRC should be considered.

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