Introduction
In parallel to minimal invasive surgery trend in urological surgery, flexible ureterorenoscopy (f-URS) and laser lithotripsy gained a significant popularity in the treatment of renal stones. Despite the technological advancements in optical and lithotripsy systems there are still some limiting factors including stone burden and abnormal renal anatomy [1]. To standardize reporting the outcomes in scientific studies and pre-operative counseling of patients some scoring systems were developed but none of them gained popularity and were not widely used. The ideal nomogram should include all the variables that might affect the outcome, besides should be simple, accurate and easy to apply in office conditions. The success of a f-URS can not only be evaluated with stone-free rates, but the ideal success should be stone-free rates without any complication or with acceptable complication rates. So, the ideal scoring system should predict the complications besides the stone free-rates. The first scoring system for predicting success of f-URS was developed by Resorlu et al. in 2012 [2].After than modified Seoul National University Stone Complexity score (S-ReSC) [3], a nomogram by Ito et al. [4] and R.I.R.S. scoring system [5] were developed. We aimed to externally validate and compare these four scoring systems for predicting capabilities of both the stone-free status and complications in a multi-institutional study.