1 3 Int Urol Nephrol DOI 10.1007/s11255-016-1311-2 UROLOGY - ORIGINAL PAPER External validation of Resorlu–Unsal stone score as predictor of outcomes after retrograde intrarenal surgery Stavros Sfoungaristos 1 · Ofer N. Gofrit 1 · Ioannis Mykoniatis 2 · Ezekiel H. Landau 1 · Ioannis Katafigiotis 3 · Dov Pode 1 · Constantinos A. Constantinides 3 · Mordechai Duvdevani 1 Received: 8 March 2016 / Accepted: 29 April 2016 © Springer Science+Business Media Dordrecht 2016 Introduction Retrograde intrarenal surgery (RIRS) represents a well- established, minimally invasive procedure for the treatment of renal stones, and it may stand as an alternative to extra- corporeal shockwave lithotripsy (SWL) and percutaneous nephrolithotomy (PCNL). PCNL provides great stone-free rates; however, this is counterbalanced by a greater dan- ger for more severe complications [1]. SWL and RIRS provide a similar safety and efficiency profile, but their outcomes are lower compared to PCNL [2]. Late techno- logical advances have contributed to increased stone-free rates, especially in the treatment of lower pole stones [3]. Although 2 cm was considered traditionally as the upper limit of stone size for RIRS [4], there are recent reports suggesting that RIRS may be an efficient treatment option even for large >2 cm renal stones, providing high stone- free rates and minimal complications [5, 6]. Preoperative assessment of treatment success may sig- nificantly contribute to optimal treatment selection and patients’ counseling. Toward this concept, several nomo- grams have been developed to facilitate preoperative pre- diction of postoperative stone-free rates after PCNL and SWL [7–11]. In 2012, Resorlu et al. [12] published the Resorlu–Unsal stone score (RUSS). RUSS is composed of four different parameters, and each of them adds 1 point to the final score. Higher scores reflect more complex cases with lower probabilities for postoperative treatment success. One point is added in patients with stone size >20 mm, lower calyceal stones and infundibulo-pelvic angle, stone number >1, and abnormal anatomy. RUSS cat- egorizes patients into four distinct groups and aims to pre- dict stone-free rates after RIRS. Although RUSS demon- strated great predictive ability in the results of the primary study, the nomogram is still missing external validation. Abstract Purpose To externally validate Resorlu–Unsal stone score (RUSS) and to evaluate its predictive accuracy. Methods Data of patients who underwent retrograde intra- renal surgery (RIRS) between October 2013 and June 2015 were collected. RUSS was applied to all patients, and the nomogram was externally validated. Area under the curve (AUC) was used for clinical validity assessment. Results A total of 85 patients were included in the study. Mean patient age was 54.3 ± 16.5, and mean stone size was 12.0 ± 6.21 mm. After applying RUSS, 56.5, 28.2, 9.41, and 5.88 % had score 0, 1, 2, and 3, respectively. RUSS was significantly associated with stone location and size. Postoperative stone-free rate was 74.1 %. Postopera- tive outcomes were significantly associated with RUSS and stone size. RUSS was found to be the only significant inde- pendent predictor in multivariate analysis, while it provided high predictive accuracy with an estimated AUC of 0.707. Conclusions RUSS is a simple scoring system that may predict postoperative stone-free rate after RIRS with great efficacy and accuracy. Keywords Ureteroscopy · Renal stone · Urolithiasis · Ureteral stones * Stavros Sfoungaristos sfoungaristosst@gmail.com 1 Department of Urology, Hadassah Hebrew University Medical Center, Ein-Kerem, 91120 Jerusalem, Israel 2 1st Urology Department, G. Gennimatas Hospital, Aristotle University, Thessaloniki, Greece 3 1st Urology Department, Laiko Hospital, University of Athens, Athens, Greece