The Cumulated Stone Diameter: A Limited Tool for Stone Burden Estimation

To compare the performance of the main methods used to estimate stone burden in order to improve and standardize preoperative evaluation of stone disease.

From January 2012 to June 2013, a series of consecutive retrograde intrarenal stone surgery was prospectively evaluated at a single institute. All patients had a pre- and postoperative CT scan. The stone burden was estimated using 3 methods: the cumulative stone diameter (M1), Ackermann's formula (M2), and the sphere formula (M3). The predictive value of the postoperative stone-free status of these methods was then compared.

Overall (n = 142), the stone-free rate was 64%. The three methods (M1, M2, and M3) were all predictive of stone-free status for stones <20 mm (P = .0076, .018, and .016, respectively). However, only M2 and M3 were significant for stones >20 mm (P = .0024 and .023). Using receiver operating characteristic curves, we found that areas under the curve were 0.73, 0.70, and 0.71 for stones below 20 mm and 0.53, 0.74, and 0.74 for stones above 20 mm for M1, M2, and M3, respectively. In multivariate analysis, M1 and M3 were both found to be independently associated with the stone-free status in the whole series (P <.001 and .011, respectively). However, for stones above 20 mm, only M3 was significant (P = .020).

Evaluation of the stone burden is an important predictor of the outcome of retrograde intrarenal stone surgery. For stones below 20 mm, all the three methods approximate stone burden correctly; however, for the stones above 20 mm, calculation of volume is recommended.