Urinary obstruction: pitfalls in the use of delayed contrast material washout for diagnosis.

Delayed washout of contrast material from the renal collecting system has been described as a reliable sign of urinary tract obstruction. However, this finding may appear in the absence of obstruction. Experiments were performed with cylinders containing meglumine iothalamate, which were perfused with water and imaged. The effects of (a) varying flow rate, (b) different volumes of the cylinders, and (c) presence of obstruction on the rate of contrast material washout were observed. In addition, postoperative urography and ureteral perfusion studies in 6 patients who had had ureteropelvic junction obstruction successfully treated by pyeloplasty were reviewed. In the experiments, it was found that changes in flow rate and volume of the reservoir affected contrast material washout; however, the presence of obstruction did not cause a delay in washout. Three of the 6 patients had delayed washout of contrast material on postoperative urographic studies, although ureteral perfusion studies had excluded obstruction. Additional mechanisms that may affect the washout of contrast material are discussed.