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.