Development and internal validation of a nomogram for predicting stone-free status after flexible ureteroscopy for renal stones.

Abstract

To develop and internally validate a preoperative nomogram for predicting stone-free status (SF) after flexible ureteroscopy (fURS) for renal stones, as there is a need to predict the outcome of fURS for the treatment of renal stone disease. We retrospectively analysed 310 fURS procedures for renal stone removal performed between December 2009 and April 2013. Final outcome of fURS was determined by computed tomography 3 months after the last fURS session. Assessed preoperative factors included stone volume and number, age, sex, presence of hydronephrosis and lower pole calculi, and ureteric stent placement. Multivariate logistic regression analysis with backward selection was used to model the relationship between preoperative factors and SF after fURS. Bootstrapping was used to internally validate the nomogram. Five independent predictors of SF after fURS were identified: stone volume (P < 0.001), presence of lower pole calculi (P = 0.001), operator with experience of >50 fURS (P = 0.026), stone number (P = 0.075), and presence of hydronephrosis (P = 0.047). We developed a nomogram to predict SF after fURS using these five preoperative characteristics. Total nomogram score (maximum 25) was derived from summing individual scores of each predictive variable; a high total score was predictive of successful fURS outcome, whereas a low total score was predictive of unsuccessful outcome. The area under the receiver operating characteristics for nomogram predictions was 0.87. The nomogram can be used to reliably predict SF based on patient characteristics after fURS treatment of renal stone disease.