Development and validation of a simple stone score (SSS) to estimate the probability of residual stones prior to percutaneous nephrolithotomy.

BACKGROUND The purpose is to develop and internally validate a simple stone score (SSS) to estimate the probability of clinically significant residual fragments (CSRF) prior to percutaneous nephrolithotomy (PNL). METHODS The files of 1170 PNL procedures between January and December 2015 were evaluated. CT-derived stone characteristics were examined. Caliceal stone distribution (CSD) was assigned three grades based on the number of calices involved regardless of the renal pelvis (I: no or single calix; II: more than one calix; III: more than 2 calices or complete staghorn stones). CSRF was defined as any residuals >4 mm in postoperative CT. A logistic regression model to predict the CSRF was fitted and coefficients were used to develop the SSS. The SSS was validated by discrimination, calibration, and decision curve analysis (DCA). RESULTS Patients' data were split into training (936, 80%) and validating (234, 20%) datasets. In the training partition, independent predictors of CSRF were CSD-grade II (OR: 4.2; 95%CI: 2.5-7; p<0.001), grade III (OR: 7.8; 95%CI: 4.2-14.4; p<0.001) and largest stone diameter (LSD) (OR:1.3; 95%CI: 1.1-1.6; p<0.001). Score points 0, 1, 2, and 0, 3, 9 were given to LSD <30, 30- 40, >40 mm, and CSD grades I, II, III, respectively. Discrimination of the SSS was 0.79 and after 10-fold cross-validation and internal validation was 0.86. The calibration plot and DCA highlighted the validity and clinical significance of the SSS. CONCLUSIONS The novel SSS could be used to describe the risk of CSRF prior to PNL. Further studies are invited for external validation.