STONE Score (PCNL Complexity)
Select CT-based parameters to calculate the STONE score, predicting percutaneous nephrolithotomy complexity and stone-free rates.
Stone size (cross-sectional area)
Measure largest stone cross-sectional area on axial CT.
Tract length (skin-to-stone distance)
Measure along planned percutaneous access tract.
Hydronephrosis
Number of calyces involved
Stone density (Hounsfield units)
Average attenuation measured on non-contrast CT.
STONE total score
5
Low complexity (STONE 5–6)
Estimated stone-free rate: 87%
How to Use This Calculator
Review preoperative CT images
Measure stone size (area), skin-to-stone distance, hydronephrosis grade, number of calyces with stone burden, and mean stone density.
Select the applicable categories
Each parameter contributes 1–3 points. The sum (5–15) provides the STONE score for PCNL complexity.
Use the score for operative planning
Higher scores signal more challenging procedures and lower stone-free rates, guiding counselling, operative time, and resource planning.
Formula
S (Stone size): <400 mm² = 1 point, 400–799 = 2, ≥800 = 3.
T (Tract length): <100 mm = 1, 100–149 = 2, ≥150 = 3.
O (Obstruction): None/mild hydronephrosis = 1, moderate = 2, severe = 3.
N (Number of involved calyces): 1 calyx = 1, 2 calyces = 2, ≥3 calyces = 3.
E (Essence/density): <950 HU = 1, 950–1200 = 2, >1200 = 3.
Full Description
The STONE nephrolithometry score integrates CT parameters to estimate PCNL complexity and the likelihood of achieving stone-free status. Higher scores correlate with longer operative time, increased blood loss, and lower stone-free rates. Use the score to counsel patients, prepare for multi-tract access or staged procedures, and allocate operative resources appropriately.
Frequently Asked Questions
How do I calculate stone area?
Measure maximum stone length and width on axial CT and multiply (L × W). Some centres use π × (L/2) × (W/2) for ellipsoid stones.
Does the STONE score replace clinical judgement?
No. It complements surgeon expertise, patient comorbidities, and anatomy. Use alongside factors such as infection status and prior surgeries.
Can it guide choice between PCNL and other modalities?
Yes. Extremely high scores may prompt consideration of staged procedures, combined modalities, or alternative approaches (e.g., RIRS, ECIRS).
Is the score validated?
Yes, the STONE score is validated in multiple cohorts to predict stone-free rates and complications after PCNL.