Bidirectional Relationship Between Systemic Immune-Inflammation Index and Postoperative Pneumonia in Elderly Patients Undergoing Colorectal Cancer Surgery: A Retrospective Cohort Study.

[OBJECTIVE] To explore the bidirectional relationship between the systemic immune-inflammation index (SII) and postoperative pneumonia (POP) in elderly patients with colorectal cancer (CRC), as well as its impact on clinical prognosis, in order to provide evidence for perioperative evidence-based management.

[METHODS] Clinical data of 2500 patients aged ≥60 years who underwent CRC resection at two hospitals from August 2017 to August 2023 were retrospectively analyzed. Demographic characteristics, preoperative/postoperative SII ((platelet × neutrophil)/lymphocyte counts), laboratory indicators, intraoperative variables, and prognosis were collected. Univariate and multivariate logistic regression analyses were used to determine the bidirectional association between SII and POP. Propensity score matching (PSM) with a 1:3 ratio was performed to balance confounding factors and verify result robustness, and the variance inflation factor (VIF) was used to assess multicollinearity.

[RESULTS] The incidence of POP was 6.8% (171/2500), and 25.0% (625/2500) of patients had a postoperative SII increase of ≥30%. Multivariate analysis showed that the highest quartile of preoperative SII (SII_Q4) was an independent risk factor for POP (OR=6.017, 95% CI: 3.377-10.72, P<0.001), and POP was independently associated with a postoperative SII increase of ≥30% (OR=9.063, 95% CI: 4.933-18.696, P<0.001). After PSM, the risks of complications such as respiratory failure, septic shock, and anastomotic leakage, as well as the 2-year mortality rate (76.6% vs 26.5%; OR=9.078), were significantly higher in the POP group than in the non-POP group (all P<0.05). The maximum VIF was 1.30, indicating no significant multicollinearity.

[CONCLUSION] Elevated preoperative SII has a bidirectional association with POP in elderly CRC patients-high preoperative SII increases POP risk, while POP exacerbates postoperative SII elevation, both leading to poor prognosis. Integrating routine perioperative SII screening and dynamic monitoring into clinical practice can facilitate early risk stratification and targeted interventions, thereby optimizing perioperative care quality and improving patient outcomes.