Estimation of size of distal ureteral stones: noncontrast CT scan versus actual size.

OBJECTIVES: To determine the accuracy of stone size determination by computed tomography (CT) for distal ureteral calculi.

METHODS: A total of 76 patients who presented with distal ureteral stone were evaluated with CT. Of the 76 patients, 41 successfully passed the stone spontaneously and intact. The largest stone size dimension measured on CT by an experienced endourologist was compared with the actual measured size of the spontaneously expulsed stone as measured by digital calipers.

RESULTS: The mean actual size of the stone was 4.51 mm (range 1-9.2, SD +/- 1.91). The mean largest dimension measured on CT was 5.51 mm (range 2-11.2, SD +/- 2.09). The correlation coefficient (r) when comparing the actual size with the CT scan measurement was -0.022. When the largest stone dimension was obtained on the craniocaudal CT images, the correlation coefficient was 0.097. When the largest stone dimension was obtained on the axial CT image, the correlation coefficient was -0.107.

CONCLUSIONS: CT scan imaging is a poor predictor of the largest stone dimension for distal ureteral calculi. Caution should be used in counseling patients on the rate of spontaneous stone passage when determined using CT measurements of stone size.