To evaluate the diagnostic value of the renal parenchymal density difference, detected using unenhanced helical computed tomography, as a secondary sign of acute obstruction due to a ureteral stone.

Fifty-five patients with acute flank pain, in whom a ureteral stone was detected on the symptomatic side using unenhanced helical computed tomography, and 22 control subjects with no urinary stone disease were included in this prospective study. Computed tomography was performed, starting from the upper poles of the kidneys down to the base of the urinary bladder. The renal parenchymal density was measured in the upper, middle, and lower portions of each kidney, and a mean value was calculated. The difference between the mean values of the two kidneys was used to predict the presence of an acutely obstructing ureteral stone.

In 49 patients with a ureteral stone (89.1%), the difference between the parenchymal densities of the obstructed and nonobstructed kidneys was 5 Hounsfield units (HU) or greater and was lower on the obstructed side. In the remaining 6 patients (10.9%), the density difference was less than 5 HU but was still lower on the obstructed side. All subjects in the control group had a density difference of less than 5 HU. A renal parenchymal density difference of 5.0 HU or greater had 89.1% sensitivity, 100% specificity, 100% positive predictive value, 85.7% negative predictive value, and 93.4% accuracy in predicting the presence of an acute obstructing ureteral stone.

These data suggest that the renal parenchymal density difference may be a valuable secondary sign of acute obstructing ureteral stone disease.