Development and internal validation of a preoperative score for predicting postoperative residual stone after mini-percutaneous nephrolithotomy.

objectiveTo develop and internally validate a simple preoperative scoring system for predicting postoperative residual stone after mini-percutaneous nephrolithotomy (mini-PCNL).

methodsThis retrospective single-center study included 217 adult patients who underwent pure mini-PCNL between May 2024 and December 2025. Postoperative stone clearance was assessed exclusively by non-contrast computed tomography. Clinically significant residual stone was defined as any residual fragment larger than 4 mm. Patients with no residual fragment or residual fragments ≤ 4 mm were classified as having no clinically significant residual stone. Univariable and multivariable logistic regression analyses were performed to identify independent preoperative predictors of residual stone. Based on the final multivariable model, a simplified preoperative scoring system, the Mini-PCNL Residual Stone Score, was constructed. Discriminative ability was evaluated using receiver operating characteristic (ROC) analysis, and internal validation was performed with bootstrap resampling. The score was also compared with the S.T.O.N.E. nephrolithometry score and Guy's Stone Score in the same cohort.

resultsOverall, 178 patients (82.0%) had no clinically significant residual stone, whereas 39 patients (18.0%) had clinically significant residual stone. Higher body mass index, greater stone number, larger stone volume, higher stone density, and the presence of a lower pole stone were independently associated with postoperative clinically significant residual stone. The Mini-PCNL Residual Stone Score ranged from 0 to 19 points and showed strong discrimination, with an AUC of 0.961. The optimal cut-off value was ≥ 10 points, corresponding to a sensitivity of 87.2% and a specificity of 93.3%. Internal validation demonstrated stable performance, with a bootstrap 95% confidence interval for the AUC of 0.929-0.986. After bootstrap correction for optimism, the optimism-corrected AUC was 0.954. In an exploratory comparison, the Mini-PCNL Residual Stone Score showed numerically higher discrimination than the S.T.O.N.E. nephrolithometry score and Guy's Stone Score.

conclusionThe Mini-PCNL Residual Stone Score is a simple and clinically applicable preoperative tool for estimating clinically significant residual stone risk after mini-PCNL. Because it is based entirely on routinely available preoperative variables, it may support individualized counseling and treatment planning. External validation in independent multicenter cohorts is needed before broader clinical use.