Acute ureteral obstruction: value of secondary signs of helical unenhanced CT.

OBJECTIVE: The purpose of our study was to determine the value of secondary signs of ureteral obstruction on helical unenhanced CT. MATERIALS AND METHODS: Over a 19-month interval, 312 patients with acute flank pain were imaged with helical unenhanced CT. Ureteral stone disease was confirmed to be present in 109 patients and confirmed to be absent in 111 patients Ninety-two remaining patients had no confirmatory imaging studies or surgery and were unable to be contacted for follow-up. For each of the 220 patients with a confirmed diagnosis, we determined the presence or absence of ureteral or collecting system dilatation, perinephric stranding, symmetry of renal size, and renal stones. In all patients with a ureteral stone, we noted the presence or absence of a circumferential rim of soft-tissue attenuation ("tissue-rim" sign) surrounding each stone and each phlebolith. RESULTS: The sensitivity of each secondary sign was ureteral dilatation, 90%; perinephric stranding, 82%; collecting system dilatation, 83%; and renal enlargement, 71%. The specificity of each secondary sign was ureteral dilatation, 93%; perinephric stranding, 93%; collecting system dilatation, 94%; and renal enlargement, 89%. Ureteral dilatation and perinephric stranding were both present or both absent in 181 of the 220 patients with a confirmed diagnosis. In this subgroup, this combination of signs had a positive predictive value of 99% and a negative predictive value of 95%. The odds ratio for the frequency of the tissue-rim sign with stones versus tissue-rim with phleboliths was 31:1. CONCLUSION: When using unenhanced CT to help diagnose acute flank pain, if one does not see a ureteral stone or does see an indeterminate but suspicious calcification, then secondary signs of obstruction are important for diagnosis. The results of this study form the basis of an imaging algorithm that can be used when interpreting unenhanced CT images of patients with acute flank pain.