Prognostic Value of the Systemic Immune-Inflammation Index (SII) for short-term clinical outcomes in the emergency department: a retrospective cohort study

BACKGROUND: The Systemic Immune-Inflammation Index (SII = PLT × NE# / LY#) is a composite haematological biomarker derivable from a routine complete blood count (CBC) that integrates neutrophil, platelet, and lymphocyte counts into a single value reflecting systemic inflammatory status. OBJECTIVE: Studies simultaneously evaluating SII across multiple concurrent outcomes in an unselected ED population remain limited. We aimed to evaluate SII's independent prognostic value for hospitalisation, ICU admission, clinical deterioration, in-hospital mortality, and 72-hour ED return, and its incremental value over NEWS2, NLR, and PLR. METHODS: Retrospective cohort study of 6,739 adults presenting to a tertiary ED (January 2022-December 2023) with a CBC at presentation. SII, NLR, PLR, and NEWS2 were calculated from admission data. Primary outcome was hospitalisation; secondary outcomes were ICU admission, clinical deterioration, in-hospital mortality, and 72-hour ED return. Multivariable logistic regression and ROC analysis were performed. RESULTS: Median SII was 768.1 (IQR: 442-1,532); 22.6% were hospitalised. SII independently predicted hospitalisation (aOR = 1.672, 95% CI: 1.551-1.803, p < 0.001; per one ln-unit increase in SII, equivalent to approximately a 2.7-fold increase in raw SII value); however, standalone discriminative performance was modest (AUC = 0.640) and should not be interpreted as clinically actionable in isolation. The primary clinical contribution was incremental: adding SII to NEWS2 increased the AUC from 0.717 to 0.758 (DeLong p < 0.001), with both markers retaining independent significance after mutual adjustment. Discriminative performance for secondary outcomes was limited (ICU admission AUC 0.430; clinical deterioration AUC 0.561; 72-hour return AUC 0.581), and SII did not independently predict in-hospital mortality after adjustment (AUC 0.571; p = 0.222). CONCLUSION: The principal contribution of SII in this study is its incremental prognostic value when combined with NEWS2, rather than standalone discrimination. SII may serve as a practical adjunct to established early warning scores during ED triage. Prospective multicentre validation is required before clinical implementation.