Performance of the systemic immune-inflammation index in predicting complicated disease and 30-day mortality in acute diverticulitis

This study evaluated whether the systemic immune-inflammation index (SII), calculated from admission blood counts, predicts complicated acute diverticulitis, intensive care unit (ICU) admission, and 30-day mortality. In this single-center retrospective observational study, 182 adults with computed tomography-confirmed acute diverticulitis and 182 age-matched controls were analyzed. Admission neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), and SII were calculated. Associations with clinical severity, complications, ICU admission, readmission, and mortality were assessed using non-parametric tests, Spearman correlation, and receiver operating characteristic analysis. White blood cell count, neutrophil count, C-reactive protein (CRP), NLR, PLR, and SII were higher in patients than in controls, whereas lymphocyte count was lower (all p < 0.001). SII correlated strongly with NLR (rho = 0.942), PLR (rho = 0.875), and CRP (rho = 0.673; all p < 0.001), and was also associated with abscess size, length of stay, and Clavien-Dindo grade. For 30-day mortality, SII yielded the highest discrimination (area under the curve [AUC] = 0.890, 95% confidence interval [CI]: 0.812–0.968). At a cut-off of 1917.74, sensitivity was 88.9% and specificity was 77.4%. Its performance was more limited for complicated disease (AUC = 0.635) and ICU admission (AUC = 0.678). Admission SII reflects inflammatory burden and adverse clinical course in acute diverticulitis. It appears most useful for short-term mortality risk stratification, but should complement rather than replace clinical assessment and imaging.