Evaluation of the STONE nephrolithometry score in predicting surgical outcomes of percutaneous nephrolithotomy: results of a prospective study at a university hospital

Objective: One of the popular advances in percutaneous nephrolithotomy (PCNL) includes nephrolithometry classification systems. It enables better patient counseling, surgery planning, outcome evaluation, and uniform academic reporting. The STONE nephrolithometry is a validated quantitative scoring system that is undervalued in clinical settings, and this study evaluates the scoring system's ability to predict the outcome of PCNL surgery. Methodology: From January 2017 to June 2018, a total of 102 PCNL patients were studied prospectively. The STONE score was derived from a preoperative non-contrast computed tomography (NCCT) scan which was used to evaluate stone-free status at 4 weeks followup. Results: The STONE nephrolithometry scoring system predicted stone-free rate (SFR) following PCNL surgery with an accuracy of 88%. The statistical cut off level of the STONE score of 8 was superior for predicting SFR. Individual variables such as stone size, degree of pelvicalyceal obstruction, number of calyceal involvement, and stone density were found to have a significant correlation with STONE score, although there was no statistically significant correlation between SFR and tract length (p=0.81). The score was divided into three categories: low complexity score 5-6 (SFR-58.7%), moderate complexity score 7-8 (SFR-40%), and high complexity score 9-13 (SFR- 1.2%). The STONE score had excellent inter-observer reliability and reproducibility (p=<0.001). Conclusions: The STONE score was a simple and easy to apply tool for predicting the stone complexity and stone clearance after PCNL. The STONE score had no statistically significant correlation with postoperative complications. Furthermore, it demonstrated high inter-observer reliability and reproducibility.