The relationship between intra-parenchymal renal resistive index variation and renal functional reserve in healthy subjects

BACKGROUND Renal functional reserve can be used as a clinical tool for risk stratification of patients undergoing potentially nephrotoxic procedures. Ultrasound assessment of intra-parenchymal renal resistive index variation-IRRIV test-has been recently proposed as a safe, reproducible, inexpensive and easy to perform technique to identify the presence of renal functional reserve. The present study has been designed to externally validate the IRRIV test in a validation cohort of healthy subjects. METHODS We examined data from a group of 47 healthy subjects undergoing protein loading and IRRIV testing. The correlation between IRRIV and renal functional reserve was tested using Pearson correlation analysis. Concordance between presence of renal functional reserve (i.e. a value of renal functional reserve ≥ 15 ml/min/1.73 m2) and IRRIV was evaluated using logistic regression analysis. RESULTS We found a significant correlation between IRRIV and renal functional reserve (Pearson correlation coefficient = 0.83 [95% confidence interval (CI) 0.71-0.90; p < 0.01]). Concordance between the presence of renal functional reserve and the IRRIV test was described in 45 (95.7%) subjects. In particular, a negative IRRIV test correctly predicted the absence of renal functional reserve in 5 subjects, while a positive IRRIV test correctly predicted the presence of renal functional reserve in 40 subjects. Only two subjects were incorrectly classified by the IRRIV test. IRRIV predicts renal functional reserve with a ROC-AUC of 0.86 [CI 95% 0.68-1]. CONCLUSIONS The IRRIV test is an ultrasound technique that significantly predicts the presence and the degree of renal functional reserve in healthy subjects.