Relationship between the intrarenal resistive index and split renal function in patients with solitary kidneys.

Background: The intrarenal resistive index (RI) is a reliable and well-established indicator for predicting total renal function in patients with two kidneys. However, it is not yet known whether the ultrasound (US) parameters, including the intrarenal RI, can be used to predict split renal function in solitary kidneys. This study aimed to investigate the relationship between US parameters, such as the RI and split renal function, in patients with solitary kidneys. Methods: A retrospective cross-sectional study was conducted of patients suspected of having solitary kidneys. From January 2004 to December 2023, the data of 78 patients who underwent US examinations for this condition at our hospital were reviewed. Renal function was evaluated using the estimated glomerular filtration rate (eGFR), calculated with the Chronic Kidney Disease Epidemiology Collaboration equation. US parameters, including kidney length and renal RI, were recorded. The study cohort was divided into two groups based on the eGFR results: the renal dysfunction (RDF) group (eGFR <60 mL/min per 1.73 m2), and the non-RDF group (eGFR ≥60 mL/min per 1.73 m2). The clinical characteristics and US parameters between these groups were compared. A logistic regression analysis was conducted to examine the relationships between the US parameters, such as the intrarenal RI and split renal function. Results: Ultimately, 57 patients with solitary kidneys [average age: 60.05±14.21 years (range, 31-86 years); 36 men] were included in the study. The multivariate regression analysis indicated that the RI and kidney length were independent predictors of RDF (P=0.005 and P=0.029, respectively). The receiver operating characteristic (ROC) curve analysis revealed area under the receiver operating characteristic (AUROC) curve values of 0.815 for the RI, 0.776 for kidney length, and 0.782 for the RI and kidney length combined (all P<0.0001). An RI cutoff value of 0.63 had a sensitivity of 85.71% and a specificity of 72.22%, while a kidney length cutoff value of 10.6 cm had a sensitivity of 61.9% and a specificity of 88.89%. The combination of both had a sensitivity of 61.9% and a specificity of 94.44%, as well as the highest accuracy of 82.46%. However, no significant differences were found among the AUROC values (all P>0.05). Conclusions: An intrarenal RI of 0.63 or higher and a kidney length of 10.6 cm or shorter predict split kidney dysfunction in patients with solitary kidneys.