Resistive index in febrile urinary tract infections: predictive value of renal outcome

In the absence of specific symptomatology in children, the early diagnosis of acute pyelonephritis is a challenge, particularly during infancy. In an attempt to differentiate acute pyelonephritis from lower urinary tract infection (UTI), we measured intrarenal resistive index (RI). We evaluated its ability to predict renal involvement as assessed by dimercaptosuccinic acid (DMSA) scintigraphy. In total 157 patients admitted to the pediatric department of the Şişli Etfal Hospital with clinical signs of febrile UTI were included in the study. The children were divided into groups according to their age at the time of ultrasonography (US). RI was measured from the renal arteries with Doppler US in the first 72 h in all 157 children. Renal involvement was assessed by ^99mTc-DMSA scintigraphy in the first 7 days after admission. The examination was repeated at least 6 months later if the first result was abnormal. All available patients with an abnormal scintigraphy underwent voiding cystourethrography 4–6 weeks after the acute infection. All patients with vesicoureteral reflux and scarred kidneys were excluded from the study. DMSA scintigraphy demonstrated abnormal changes in 114 of 157 children and was normal in the remaining 43 children. Of these 114 children, 104 underwent repeat scintigraphy, of whom 77 showed partially or totally reversible lesion(s). Of these 77 children, 17 children (22%) with vesicoureteral reflux were excluded. Thus, we compared the 43 children with lower UTI with the 60 children with definite acute pyelonephritis at admission. Kidneys with changes of acute pyelonephritis had a mean RI of 0.744±0.06 in infants, 0.745±0.03 in preschool children, and 0.733±0.09 in patients of school age with upper UTI. However, the mean RI was 0.703±0.06 in infants, 0.696±0.1 in preschool children, and 0.671±0.09 in school-aged patients with lower UTI. The mean RI values were significantly higher in patients with upper UTI (P<0.001). There was a highly significant correlation between RI values and the severity of the renal lesion as ranked by DMSA scintigraphy (P<0.001). When the cut-off RI value was 0.715, there was an 80% sensitivity and a 89% specificity for diagnosing upper UTI. Refluxing kidneys and scarred kidneys also had higher RI values. In conclusion, RI values were increased significantly in children with febrile UTI when renal parenchymal involvement (assessed by DMSA scintigraphy) was present. Our results also support the view that the children with high RI values are at a high risk of reflux, scarring, or both, which was frequently observed in febrile UTI. This might allow identification of patients at risk for severe renal lesions that require more aggressive therapy, investigation, and follow-up than those with lower UTI.