Preoperative neutrophil-to-lymphocyte ratio and systemic immune-inflammation index as prognostic biomarkers for postoperative pneumonia and pulmonary complications after thoracic surgery: a systematic review and meta-analysis.

Background: Postoperative pneumonia and pulmonary complications are major adverse events following thoracic surgery. The neutrophil-to-lymphocyte ratio (NLR) and systemic immune-inflammation index (SII), two circulating immune-inflammatory biomarkers derived from routine complete blood counts, have been individually associated with postoperative outcomes, but no quantitative synthesis has specifically evaluated their prognostic value for pulmonary outcomes after lung resection or esophagectomy. Methods: A systematic search of PubMed, Embase, Web of Science, Scopus, CENTRAL, CNKI, and Wanfang Data was conducted from inception to 11 April 2026. Observational studies reporting associations between preoperative NLR and/or SII and postoperative pneumonia or pulmonary complications in adults undergoing lung resection or esophagectomy were eligible. A hierarchical framework prioritized continuous associations as the primary synthesis, with categorical high-versus-low comparisons as secondary. Random-effects models with restricted maximum likelihood estimation and the Hartung-Knapp adjustment were used, and certainty of evidence was assessed using GRADE. Results: Eight retrospective cohort studies comprising 3,936 patients were included. Higher preoperative NLR and SII showed directionally consistent associations with increased postoperative pulmonary risk across all analyses. For NLR, continuous analyses suggested an approximately twofold increase in the odds of postoperative pneumonia per one-unit increment, and categorical analyses an approximately fourfold increase above study-specific thresholds. For SII, the continuous association was more modest, while categorical analyses indicated more than twice the odds above study-specific cut-offs. However, Hartung-Knapp confidence intervals crossed unity in three of the four pooled analyses, reflecting the very small number of contributing studies (k = 2-3 per analysis), and the overall certainty of evidence was low to very low. Conclusion: Higher preoperative NLR and SII may be associated with an increased risk of postoperative pneumonia and related pulmonary complications after thoracic surgery, but the evidence base remains small, exclusively retrospective, and geographically concentrated. These low-cost immune-inflammatory indices should be regarded as promising, hypothesis-generating prognostic markers that require confirmation in larger, prospective, and methodologically standardized studies before they can be considered for routine perioperative risk stratification. Systematic review registration: https://www.crd.york.ac.uk/PROSPERO/view/CRD420261329394, identifier PROSPERO (CRD420261329394).