Renal tract calculi: comparison of stone size on plain radiography and noncontrast spiral CT scan

Noncontrast spiral CT (NCCT) has emerged as the investigation of choice in patients presenting with renal-tract calculi. As management guidelines are based on stone size measured on plain radiography of the kidneys, ureters, and bladder (KUB), it is important to assess the accuracy of stone size measured on NCCT compared with KUB films.

The NCCT and KUB studies obtained from 24 patients (27 stones) presenting to the emergency department at a major metropolitan hospital were analyzed randomly and independently by two urologists and one uroradiologist. The NCCT scans were assessed separately from the KUB films. Only size in greatest dimension and stone location were recorded.

The stone size was 2 to 38 mm on NCCT scans and 2 to 46 mm on KUB films. The mean stone size was 6.773 +/- 6.146 mm and 7.747 +/- 7.866 mm, respectively (P = 0.0398; Student's t-test). Almost three fourths (70%) of the stones were larger on KUB films than they were on NCCT scans, with a mean difference -0.974 mm (95% confidence interval -5.652, 3.703) for NCCT.

Spiral CT underestimates stone size by approximately 12% compared with KUB films. This error may impact stone management as outlined in guidelines published by the American Urological Association, particularly for stones about 5 mm in greatest dimension. These patients may initially be managed conservatively when intervention would be more appropriate.