To evaluate differences in stone measurement using computed tomography (CT) and ultrasound (US). Axial unenhanced helical CT is the reference-standard imaging modality for the assessment of urinary tract calculi; however, US is also commonly used. Differences in stone measurement using these techniques are poorly described and contributors to measurement error remain unknown.

All patients at our institution undergoing both abdominal CT and renal US less than 1 month apart since June 2004 were reviewed. Solitary renal calculi were identified on both CT and US in all cases.

We identified 71 calculi in 60 patients. Compared with CT, US overestimated stone size, an effect that was more pronounced with smaller calculi. The mean stone measurement on CT was 7.4 ± 4.4 mm and on US it was 9.2 ± 4.5 mm (P = .018). For stones ≤5 mm, US measurements were a mean of 1.9 ± 1.2 mm greater than CT (P <.001). US and CT measurements were discordant for 60% of stones ≤5 mm. Discordance was associated with US measurement of skin-to-stone distance (P = .018), but not body mass index (P = .189) or location within the urinary tract (P = .161). Review of the literature revealed that US has a pooled sensitivity and specificity of 45% and 94%, respectively, for the detection of ureteric calculi and 45% and 88%, respectively, for renal calculi.

US overestimates stone size in urolithiasis, a finding that may have implications for stone management. Discordance in stone measurement varies with size and is greatest in stones ≤5 mm. US measurement of skin-stone-distance is an important determinant of error in US measurement of renal calculi.