A combined systemic immune-inflammation index and prognostic nutritional index score for predicting overall survival after resection of pancreatic ductal adenocarcinoma: a retrospective cohort study

Background The systemic immune-inflammation index (SII) and prognostic nutritional index (PNI) have been individually associated with prognosis in various cancers. This study aimed to develop and validate a combined SII-PNI-based risk score for predicting overall survival (OS) in patients with resectable pancreatic ductal adenocarcinoma (PDAC). Methods This retrospective study enrolled 375 consecutive patients with PDAC who underwent curative-intent resection at The Second Hospital of Hebei Medical University between January 2014 and July 2025. Using X-tile software, optimal cut-off values for SII and PNI were determined based on 1-year OS in a training cohort (constituting 70% of the patients). A combined risk score was constructed using Cox regression coefficients and dichotomized at the optimal cut-off (− 0.231). The model’s performance was assessed using receiver operating characteristic (ROC) curve analysis, Kaplan–Meier survival curves, and multivariate Cox regression. Results The combined SII-PNI model achieved AUCs of 0.758 in the training cohort and 0.750 in the validation cohort. Kaplan–Meier analysis showed significantly worse OS in the high-risk group (p < 0.0001). In univariate analysis, elevated SII (p = 0.044) and reduced PNI (p < 0.0001) were both significantly associated with poorer OS. Multivariate Cox regression analysis confirmed PNI (HR = 0.891, 95% CI: 0.863–0.919, p < 0.001) and age (HR = 1.029, 95% CI: 1.013–1.046, p < 0.001) as independent prognostic factors, whereas SII and sex were not. Conclusion The combined SII-PNI score is a simple, cost-effective, and powerful predictor of prognosis in resectable PDAC, enabling reliable postoperative risk stratification.