Nonlinear association between systemic inflammation response index and mortality in adult cardiac surgery–associated acute kidney injury: A retrospective Cohort study based on the MIMIC-IV database

Systemic inflammation plays a pivotal role in the pathogenesis and prognosis of cardiac surgery–associated acute kidney injury (CSA-AKI). The Systemic Inflammation Response Index (SIRI) has emerged as a novel biomarker reflecting immune inflammatory balance. This study aimed to explore the association between SIRI and in-hospital mortality among patients with CSA-AKI. Data of CSA-AKI patients were extracted from the MIMIC-IV 3.1 database. SIRI was calculated and divided into 3 tertiles. Restricted cubic spline (RCS) models were used to assess nonlinear associations between SIRI and mortality. Multivariate logistic regression models were constructed to evaluate the independent relationship between SIRI and outcomes after adjusting for potential confounders. A total of 5394 patients with CSA-AKI were included. Patients in the medium SIRI tertile (1.27–3.06) demonstrated the lowest mortality (1.3%) compared to the low (2.0%) and high (3.1%) tertiles. RCS analysis revealed a significant U-shaped association between SIRI levels and in-hospital mortality risk ( P for nonlinearity < .001). Below the inflection point (SIRI = 1.6899), mortality risk tended to decrease with increasing SIRI, while above this threshold, each 1-unit increase in SIRI was significantly associated with a 10% higher risk of in-hospital mortality (OR 1.10, 95% CI 1.03–1.16, P  = .001) after adjustment. SIRI demonstrated a U-shaped association with mortality among patients with CSA-AKI, indicating that both immune suppression and excessive inflammation are detrimental. SIRI may serve as a simple, readily available biomarker to reflect immune homeostasis and assist in postoperative risk stratification. Further prospective studies are warranted to validate these findings.