How practical is the application of percutaneous nephrolithotomy scoring systems? Prospective study comparing Guy's Stone Score, S.T.O.N.E. score and the Clinical Research Office of the Endourological Society (CROES) nomogram

Abstract

Objective: To prospectively compare the Guy's Stone Score (GSS), S.T.O.N.E. [stone size (S), tract length (T), obstruction (O), number of involved calices (N), and essence or stone density (E)] score and the Clinical Research Office of the Endourological Society (CROES) nephrolithometric nomogram to predict percutaneous nephrolithotomy (PCNL) success rate and assess the correlation with perioperative complications.

Patients and methods: We prospectively evaluated all consecutive PCNL patients at our institute between 1 November 2013 and 31 May 2015. The above scoring systems were applied to preoperative non-contrast computed tomography and the practical difficulties in such applications were noted. Perioperative complications and the stone-free rate (SFR) were also recorded. Receiver operating characteristic curves were drawn and the areas under curves were compared and appropriate statistical analysis done.

Results: In all, 48 renal units were included in the study. The overall SFR was 62.2%. The presence of staghorn stones (β = 27.285, 95% confidence interval 1.19-625.35; P = 0.039) was the only significant variable associated with the residual stones on multivariate analysis. Stone-free patients had significantly lower median GSS (2 vs 4) and S.T.O.N.E. scores (6 vs 10) and higher median CROES scores (83% vs 63%) (all P < 0.001) compared to residual-stone patients. All scoring systems were significantly associated with SFR (all P < 0.001). There was no significant difference in the areas under curves of the scoring systems (0.858, 0.923, and 0.931, respectively). Furthermore, all scoring systems had weak correlations with Clavien-Dindo classified complications (r = 0.29, P = 0.045; r = 0.40, P = 0.005 and r = -0.295, P = 0.04, respectively). We found no standardisation for the measurement of stone dimensions, tract length, Hounsfield units, and staghorn definition.

Conclusions: All scoring systems equally predicted SFR and had a weak correlation with Clavien-Dindo complications. Standardisation is needed for the variables in which they have been found deficient.

Keywords: 3D, three-dimensional; ACS, acute angle, complicated calyx and stone size; AUC, area under curve; BMI, body mass index; CCI, Charlson Comorbidity Index; CROES, Clinical Research Office of the Endourological Society; Clinical Research Office of the Endourological Society (CROES); GSS, Guy’s Stone Score; Guy’s Stone Score; HU, Hounsfield unit; IQR, interquartile range; KUB, plain abdominal radiograph of the kidneys, ureters and bladder; NCCT, non-contrast CT; PCNL, percutaneous nephrolithotomy; Percutaneous nephrolithotomy (PCNL); ROC, receiver operating characteristic; Renal stone; S.O.N., stone size, obstruction and number of involved calyces; S.T.O.N.E. score; S.T.O.N.E., stone size (S), tract length (T), obstruction (O), number of involved calices (N), and essence or stone density (E); SFR, stone-free rate; SFS, stone-free status; SPSS, Statistical Package for the Social Sciences; SSD, skin-to-stone distance; SWL, shockwave lithotripsy; US, ultrasonography.

Fig. 1

ROC curves for the scoring systems: the AUCs for GSS, S.T.O.N.E. score and CROES nomogram were 0.858, 0.923 and 0.931 (1.00–0.69), respectively. Increasing values of GSS and S.T.O.N.E. score predict poorer chances whereas increasing scores on CROES nomogram predict better chances of being stone free.

Fig. 2

(a) Dimensions of solitary oval stone reliably measured. (b) Irregular shape staghorn calculus on a coronal section of CT scan. (c) and (d) Same stone in two different axial cuts. Measurement of dimensions of this stone on axial images will produce incorrect stone size. (e) Coronal section of CT scan of another patient depicting two stones: one in pelvis and the other in the inferior calyx. (f) Calculation of tract length/SSD: the stone farthest from the skin (pelvic stone in this case) chosen from the above coronal section. On axial section, centre of the stone marked and three lines drawn at 0, 45 and 90° posteriorly intersecting the skin. These three distances measured and mean value taken. (g) Coronal section of CT scan showing two stones with different HU. Higher HU value was taken for the calculation in view of higher score (more difficult) given for >950 HU in the S.T.O.N.E. scoring system.

Fig. 3

ROC curves for S.T.O.N.E. and S.O.N. scores. The AUCs for the S.T.O.N.E. and S.O.N. scores were 0.923 and 0.922, respectively.