Utility of the resistance index ratio in differentiating obstructive from nonobstructive hydronephrosis in children

Purpose. We assessed the utility of the resistance index ratio (RIR) in distinguishing between obstructive and nonobstructive upper urinary tract dilatation in children. Methods. Twenty-three children (7 days-14 years old) with unilateral dilated collecting systems and a contralateral normal kidney were prospectively evaluated by duplex Doppler sonography. We measured the resistance index (RI) of the intrarenal arteries and calculated the RIR. Ninety-six normal kidneys in 48 controls were also evaluated. Results. Twelve kidneys were proved to be obstructed at the ureteropelvic junction, and 11 were found by renal scintigraphy with furosemide and/or by surgery to have nonobstructive dilatation. The mean RIR differed significantly between the obstructed and dilated nonobstructed kidneys (1.16 ± 0.04 versus 1.04 ± 0.04, respectively; p < 0.01). After surgical correction of ureteropelvic junction obstruction, the obstructed kidneys showed significant drops in the RIR (1.18 ± 0.03 to 1.07 ± 0.03, p < 0.01). The RIR showed no statistically significant relationship with age (r = -0.268, p > 0.01); however, the RI declined with increasing age (r=-0.414, p < 0.01). An RIR cut-off value of ≥ 1.10 provided a good discriminatory level, resulting in a sensitivity of 92%, a specificity of 97%, a positive predictive value of 85%, and a negative predictive value of 97%. Conclusions. An RIR cut-off value of ≥ 1.10 appears to be an effective parameter for evaluation and follow-up of unilateral obstructive hydronephrosis in children.