title: External validation of the S.T.O.N.E. nephrolithometry scoring system.
abstract: BACKGROUND
  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: 250-4030), 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).
  
  
  CONCLUSIONS
  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.
authors.affiliation: Division of Urology, McGill University Health Centre, Montreal QC; ; Department of Urology, Benha University, Benha, Egypt;
authors.forename: Yasser A
authors.initials: YA
authors.lastname: Noureldin
authors.affiliation: Division of Urology, McGill University Health Centre, Montreal QC; ; Department of Urology, Suez Canal University, Ismailia, Egypt.
authors.forename: Mohamed A
authors.initials: MA
authors.lastname: Elkoushy
authors.affiliation: Division of Urology, McGill University Health Centre, Montreal QC;
authors.forename: Sero
authors.initials: S
authors.lastname: Andonian