Obstruction and the minimally dilated renal collecting system: US evaluation.

To assess the significance of the ultrasonographic (US) demonstration of grade 1 hydronephrosis, the abdominal or renal sonograms of 370 patients were retrospectively reviewed. In 80 patients, there was a clinical suspicion of renal obstruction. Grade 1 hydronephrosis was observed in 33 of the 80 patients, and obstruction was confirmed in 17 of those 33 patients (51.5%). In 290 patients, US was performed for reasons other than a suspicion of renal obstruction. Grade 1 hydronephrosis was observed in 37 of the 290 patients. Three of the 37 patients were excluded from further consideration due to lack of follow-up. Obstruction was confirmed in two of the remaining 34 patients (6%). It appears that grade 1 hydronephrosis is significant when the clinical question of renal obstruction is raised. As an incidental finding, however, it is unlikely to indicate obstruction.