Objectives: To report the development and validation of a scoring system, the Guy's stone score, to grade the complexity of percutaneous nephrolithotomy (PCNL). Currently, no standardized method is available to predict the stone-free rate after PCNL.
Methods: The Guy's stone score was developed through a combination of expert opinion, published data review, and iterative testing. It comprises 4 grades: grade I, solitary stone in mid/lower pole or solitary stone in the pelvis with simple anatomy; grade II, solitary stone in upper pole or multiple stones in a patient with simple anatomy or a solitary stone in a patient with abnormal anatomy; grade III, multiple stones in a patient with abnormal anatomy or stones in a caliceal diverticulum or partial staghorn calculus; grade IV, staghorn calculus or any stone in a patient with spina bifida or spinal injury. It was assessed for reproducibility using the kappa coefficient and validated on a prospective database of 100 PCNL procedures performed in a tertiary stone center. The complications were graded using the modified Clavien score. The clinical outcomes were recorded prospectively and assessed with multivariate analysis.
Results: The Guy's stone score was the only factor that significantly and independently predicted the stone-free rate (P = .01). It was found to be reproducible, with good inter-rater agreement (P = .81). None of the other factors tested, including stone burden, operating surgeon, patient weight, age, and comorbidity, correlated with the stone-free rate.
Conclusions: The Guy's stone score accurately predicted the stone-free rate after PCNL. It was easy to use and reproducible.
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