Predictive value of secondary signs of obstruction in follow-up computed tomography of ureteral stones: a study with dynamic computed tomography.

Objectives: This study aimed to determine the ratio of obstruction and predictive values of secondary signs in follow-up computed tomography (CT) of ureterolithiasis patients; to correlate stone characteristics with obstruction; to compare enhancement of obstructed and non-obstructed kidneys; and to compare radiation dose of the dynamic CT protocol to an excretory-phase protocol.

Materials And Methods: This retrospective study assessed 49 follow-up CT scans of patients with remaining ureterolithiasis after a renal colic episode. Obstruction was measured as time taken to excretion of contrast medium in dynamic CT. Degree of secondary signs of obstruction was evaluated from the unenhanced CT. Data were collected on patients' gender and age, stone size and location, time from renal colic to follow-up, attenuation of the renal cortex and radiation dose.

Results: Obstruction was present in 28% (n = 14) at follow-up. Predictive values (sensitivity, specificity, positive predictive value, negative predictive value) were calculated for hydronephrosis (1.0, 0.63, 0.52, 1.0), hydroureter (1.0, 0.4, 0.4, 1.0), perirenal stranding (0.21, 0.94, 0.6, 0.75), Gerota's fascia (0.21, 0.97, 0.75, 0.76) and renal swelling (0.21, 0.97, 0.75, 0.76). Obstruction was not correlated with stone characteristics. Enhancement was lower in obstructed kidneys (p < 0.01). Radiation dose was reduced by 43% (1.8 mSv).

Conclusions: Obstruction was found in 28% of patients. Secondary signs were scarce and of indeterminate value to the diagnosis of obstruction. The absence of hydronephrosis and hydroureter contradicted obstruction. Stone characteristics were not correlated with obstruction. Enhancement of the renal cortex was lower in obstructed kidneys. The dynamic protocol reduced the radiation dose.