Sonography of pyonephrosis: a prospective study.

Sonograms of 73 patients with 92 hydronephrotic kidneys were prospectively reviewed in an attempt to differentiate hydronephrosis from pyonephrosis. Sonographic diagnosis of pyonephrosis was based on the presence of persistent internal echoes, dispersed or dependent, within the dilated pelvocaliceal system. In group 1, consisting of 38 patients without clinical evidence of renal infection, sonography revealed the collecting system distended by urine to be anechoic, for a specificity of 100%. In group 2, consisting of 34 patients with clinical suspicion of renal infection, sonography showed internal echoes within the fluid-filled collecting system in 10 cases; nine of these had pyonephrosis (sensitivity of 90%), and one had hemorrhagic debris without infection (false-positive rate 3%). In the other 24 patients, sonography correctly predicted the absence of infection in all but one case (specificity 97%, false-negative rate 10%). It is concluded that in patients with clinical suspicion of renal infection, sonography has a high degree of accuracy (96%) in the differentiation of pyonephrosis from hydronephrosis.