Role of the resistive index in the evaluation of acute renal obstruction.

Sonography is well established as a screening tool to evaluate the kidney for the presence of renal obstruction. The role of sonography in the evaluation of acute obstruction is less clear, however. Despite recent reports emphasizing the ability of gray-scale sonography and kidney/urinary/bladder films to screen for acute colic, experience indicates that the sonographic diagnosis of acute renal stone disease may be unreliable. Multiple studies have demonstrated the many potential pitfalls of the sonographic examination for acute obstruction. False-negative studies may result if the kidney is evaluated before hydronephrosis occurs. Conversely, nonobstructive pyelocaliectasis and parapelvic cysts may result in false-positive studies. Because of these sonographic limitations, many imaging practices exclusively have utilized excretory urography for the evaluation of patients with symptoms of renal colic.