External validation of the STONE nephrolithometry scoring system

Introduction: We perform external validation of the S.T.O.N.E. nephrolithometry scoring system for the preoperative assessment of percutaneous nephrolithotomy (PCNL) outcomes. Methods: After obtaining institutional review board approval, all PCNLs performed from 2009 to 2013 at a tertiary referral centre were reviewed. The S.T.O.N.E. score was calculated and correlated with stone-free status, estimated blood loss (EBL), operative time, length of hospital stay (LOS), and postoperative complications. Results: A total of 155 PCNLs were included, with 100 (64.5%) males and 55 (35.5%) females. The mean age was 54.9 +/- 1.2 years (range: 17-85), with a mean body mass index of 26.9 +/- 0.5 kg/m(2) (range: 17.2-51). The mean S.T.O.N.E. score was 7.67 +/- 0.1 (range: 5-12), with a mean stone size of 609.8 +/- 48.4 mm(2) (range: 2504030), a mean Hounsfield unit of 887.7 +/- 25.3 (range: 222-1766), a mean tract length of 97.3 +/- 1.9 mm (range: 53-175), a mean operative time of 100.1 +/- 2.8 min (range: 60-240), and a mean LOS of 4.2 +/- 0.3 days (range: 1-18). The overall stone-free rate after the primary procedure was 71.6%. The S.T.O.N.E. score significantly affected stone-free status (p = 0.001) and EBL (p = 0.003). There was significant correlation between the S.T.O.N.E. score and operative time (r = 0.4; p < 0.001) and LOS (r = 0.3; p = 0.001). Therefore, the higher the S.T.O.N.E. score, the longer the operative time, the higher the EBL, the longer the LOS, and the lower the chance of being stone-free. The overall complication rate after the primary procedure was 15.5%, which did not correlate with the S.T.O.N.E. score (p = 0.9). Conclusion: Although this study externally validates the S.T.O.N.E. scoring system, its accuracy is comparable to stone size and number of involved calyces in predicting stone-free status post-PCNL.