Renal Artery Resistive Index as an Early Cardiovascular–Renal Biomarker in Preterm Neonates: Differential Associations with Hypertension and Acute Kidney Injury

Background: Renal artery resistive index (RI) may reflect renal–vascular adaptation in preterm neonates, but its ability to distinguish early hypertension from acute kidney injury (AKI) remains uncertain. Methods: This prospective cohort study enrolled preterm neonates born at 28–34 weeks’ gestation at Dr. Hasan Sadikin Hospital, Indonesia. Renal Doppler ultrasonography was performed at 72 h of life to measure the RI of the right and left renal arteries. Blood pressure was assessed during the first postnatal week using repeated non-invasive oscillometric measurements. AKI was defined according to neonatal Kidney Disease: Improving Global Outcomes criteria. Associations between RI, early-onset hypertension, and AKI were evaluated using bivariate analyses, receiver operating characteristic curves, and exploratory adjusted logistic regression. Results: Of 113 eligible infants, 77 completed follow-up evaluation. Higher RI values were associated with early-onset hypertension in univariable analyses for both the right (p = 0.010) and left (p = 0.038) RI. RI thresholds > 0.94 for the right renal artery and >0.87 for the left renal artery were associated with increased risk of hypertension. RI values did not differ significantly between infants with and without AKI. Exploratory adjusted analyses showed attenuation of side-specific associations, whereas the higher bilateral RI value suggested a modest hypertension-related signal. Conclusions: In this cohort of clinically stable surviving preterm neonates, renal artery RI showed a clearer association with early-onset hypertension than with AKI classified using the study’s operational KDIGO approach. RI may provide adjunctive physiological information regarding vascular impedance and early cardiovascular–renal adaptation, but it should not be interpreted as a specific or stand-alone biomarker of hypertension or kidney injury. These findings are hypothesis-generating and require validation in larger multicenter cohorts using serial renal and hemodynamic assessments.