Renal Resistive Index on Admission Predicts and Mediates Acute Kidney Injury: A Prospective Observational Study from a Greek Intensive Care Unit

Background/Objectives: The renal resistive index (RRI) has emerged as an early marker of renal vascular resistance. The purpose of this study was to investigate the association between RRI on intensive care unit (ICU) admission and the development of acute kidney injury (AKI) in a general ICU population, and to assess its predictive accuracy. Methods: This prospective observational study was conducted in a multidisciplinary ICU. Consecutive mechanically ventilated adults were enrolled; RRI was measured within 24 h of admission after hemodynamic stabilization. AKI was defined by Kidney Disease: Improving Global Outcomes (KDIGO) criteria within seven days. Multivariable regression, receiver operating characteristic (ROC), reclassification, and mediation analyses were performed. Results: A total of 181 patients were included. AKI occurred in 36%. Median RRI was 0.73 (0.65–0.80). RRI correlated with age, acute physiology and chronic health evaluation (APACHE) II and sequential organ failure assessment (SOFA) scores, lactate, and glomerular filtration rate (GFR) (all p < 0.001). In multivariable analysis, RRI was the only independent predictor of AKI (OR 2.86 per 0.05 increase, 95% CI 1.64–4.98, p = 0.001). It was also associated with an increased likelihood of presenting with a more severe AKI stage. RRI showed high discriminative ability (AUC = 0.89, 95% CI 0.84–0.94); the optimal cut-off was 0.77 (sensitivity 0.83, specificity 0.82). Adding RRI to a clinical model improved prediction (ΔAUC p = 0.049; net reclassification index (NRI) = 0.52, p < 0.001). Mediation analyses showed that RRI significantly mediated the effects of hypertension and low baseline GFR on AKI risk. Subgroup analyses confirmed consistent predictive performance across age, lactate, and sepsis categories. Conclusions: RRI is an independent early predictor of AKI and its severity, as well as a mediator of both hypertension and low GFR, regarding their effect on AKI development in ICU patients. RRI could serve as an early bedside marker of renal perfusion impairment in critically ill patients, guiding strategies aimed at reducing the risk of AKI.