Guy's Stone Score: A Validated Grading System for Percutaneous Nephrolithotomy
The Guy's Stone Score (GSS) is a four-grade classification system that accurately predicts stone-free rates and complications following percutaneous nephrolithotomy (PCNL) based on stone complexity, location, and patient anatomy. 1
What is the Guy's Stone Score?
The GSS is a standardized scoring system developed to grade the complexity of kidney stones before PCNL and predict surgical outcomes. 1 It was created through expert consensus, literature review, and iterative testing, and has been validated as reproducible with good inter-rater agreement (kappa = 0.81). 1
The Four Grades
Grade I (Simplest):
- Solitary stone in mid or lower pole, OR
- Solitary stone in the renal pelvis with simple anatomy
- Expected stone-free rate: 76-95% 1, 2, 3
Grade II (Moderate complexity):
- Solitary stone in upper pole, OR
- Multiple stones in a patient with simple anatomy, OR
- Solitary stone in a patient with abnormal anatomy
- Expected stone-free rate: 71-79% 1, 2, 3
Grade III (Complex):
- Multiple stones in a patient with abnormal anatomy, OR
- Stones in a caliceal diverticulum, OR
- Partial staghorn calculus
- Expected stone-free rate: 53-60% 1, 2, 3
Grade IV (Most complex):
- Complete staghorn calculus, OR
- Any stone in a patient with spina bifida or spinal injury
- Expected stone-free rate: 20-41% 1, 2, 3
Clinical Uses and Predictions
Stone-Free Rate Prediction
The GSS is the only factor that significantly and independently predicts stone-free rates after PCNL (P = 0.01). 1 Importantly, other factors including stone burden alone, operating surgeon, patient weight, age, and comorbidity did not correlate with stone-free rates when GSS was accounted for. 1
There is a significant inverse correlation between GSS grade and stone-free rate - as the grade increases, the likelihood of achieving complete stone clearance decreases. 2
Complication Prediction
Complications increase significantly with higher GSS grades, particularly for grades III and IV. 2 Major complications (Clavien score 3-5) are significantly more frequent in higher GSS categories. 4
Specific outcomes that correlate with higher GSS grades include: 2, 3
- Increased blood transfusion rates (0% for Grade I vs. 22.2% for Grade IV)
- Longer operative times (63 minutes for Grade I vs. 153 minutes for Grade IV)
- Lower tubeless PCNL rates (54.8% for Grade I vs. 7.4% for Grade IV)
- Need for multiple access tracts
- Longer hospitalization duration
- Higher rates of auxiliary procedures
Preoperative Planning
The GSS can be calculated using either CT scan or intravenous pyelogram (IVP) findings. 4, 3 CT-based GSS assessment is the gold standard, as non-contrast CT is strongly recommended before PCNL. 5
The score helps surgeons: 2
- Set realistic expectations with patients about success rates
- Plan appropriate operative time and resources
- Determine need for blood product availability
- Counsel patients on complication risks
- Make informed decisions about alternative treatment approaches for very complex stones
Important Clinical Caveats
The GSS was specifically developed and validated for PCNL outcomes only - it does not apply to ureteroscopy or shock wave lithotripsy treatment planning. 1
While the original GSS has proven utility, some studies suggest modifications may improve prognostic discrimination, particularly ensuring at least 20% difference in stone-free rates between adjacent grades. 6 However, the original four-grade system remains the validated standard. 1
Patient anatomy matters significantly - abnormal anatomy (such as horseshoe kidney, ectopic kidney, or ureteropelvic junction obstruction) automatically elevates the GSS grade and worsens prognosis. 1
The GSS has been validated in both developed and developing country settings, including regions with high rates of complex stone disease, confirming its broad applicability. 2