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STONE Nephrolithometry Score

Predict PCNL outcomes using the validated STONE score. Five CT-based components—Size, Tract, Obstruction, Number, Essence—stratify stone-free rates and complication risks.

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Scores ≤5: ~93% SFR Scores 9–11: ~64% SFR Staghorn: ~45% SFR

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Why: Accurate outcome prediction helps urologists counsel patients and plan surgical approaches.

How: Sum five component scores (5–16). Lower scores predict higher stone-free rates and fewer complications.

Scores ≤5: ~93% SFRScores 9–11: ~64% SFR
Sources:AUA Kidney Stone GuidelinesEAU Urolithiasis

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🏥 Sample Clinical Scenarios — Click to Load

Stone and Anatomy Assessment

Enter stone characteristics from preoperative CT imaging

Calculated from CT dimensions (length × width × π × 0.25)
Skin-to-stone distance on CT
Degree of hydronephrosis on imaging
Number of calyces containing stone material
Hounsfield units on non-contrast CT
Overall surgical complexity assessment

For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.

🏥 Health Facts

📊

5–16 point range

— Journal of Urology

🏥

76–98% SFR for >2cm stones

— AUA/EAU

📋 Key Takeaways

  • • The STONE score is a validated nephrolithometry scoring system that predicts outcomes after percutaneous nephrolithotomy (PCNL)
  • • Scores range from 5 to 16 — lower scores predict higher stone-free rates and fewer complications
  • • Five factors are assessed: Size, Tract length, Obstruction, Number of stones, and Essence (density)
  • • Patients with scores ≤5 have stone-free rates above 90%, while scores ≥9 drop below 65%
  • • This tool helps urologists counsel patients on expected outcomes and plan surgical approaches

💡 Did You Know?

📊The STONE score was developed by Akhavein et al. in 2015 and validated across multiple international centers with over 800 patientsSource: Journal of Urology
🏥PCNL has a 76-98% stone-free rate for stones >2cm, making it the gold standard for large renal calculiSource: AUA/EAU Guidelines
💎Stones with density >950 HU (Hounsfield Units) on CT scan are harder to fragment and require more energy during lithotripsySource: Radiology Studies
📉Staghorn calculi (filling the entire renal pelvis) have stone-free rates of only 50-60% with single PCNL sessionSource: Endourology Society
🔬The most common kidney stone composition is calcium oxalate (75-80%), followed by uric acid (5-10%) and struvite (10-15%)Source: NIDDK
⚕️Kidney stones affect approximately 1 in 10 people globally, with recurrence rates of 50% within 5-10 years without preventive measuresSource: WHO Data

📖 How the STONE Scoring System Works

The STONE score uses five objective CT-based measurements to generate a composite score (5-16 points) that correlates with PCNL outcomes.

Step 1: Obtain Non-Contrast CT

High-quality non-contrast CT is essential for accurate measurements of stone size, density, and tract planning.

Step 2: Measure Each Component

S (Stone size): length × width × 0.25 × π. T (Tract): skin-to-stone distance. O (Obstruction): hydronephrosis grade. N (Number): calyces involved. E (Essence): Hounsfield units.

Step 3: Sum and Stratify

Total score 5-6 = Low complexity (~93% SFR). 7-8 = Moderate (~83%). 9-11 = High (~64%). 12+ = Very high (~45%).

🎯 Expert Tips for PCNL Planning

💡 Preoperative Optimization

Ensure sterile urine before PCNL. Treat UTI, stop anticoagulation per protocol, and review CT for calyceal anatomy.

💡 Posterior Calyx Access

Posterior calyx access is preferred to avoid major vessels. Upper pole access may require supracostal approach.

💡 Stone Density Matters

Stones >1100 HU fragment poorly. Consider laser settings and expect longer operative time for dense stones.

💡 Staged Procedures

For STONE score ≥9, plan for possible second-look procedure. Nephrostomy tube facilitates re-entry.

⚖️ STONE vs Other Scoring Systems

FeatureSTONE ScoreGuy's ScoreS.T.O.N.E.
Variables5 (S,T,O,N,E)45
CT-based✅✅✅
Stone-free prediction✅✅✅
Complication prediction✅⚠️ Limited✅
ValidationMulti-centerSingle centerMulti-center
Range5-161-45-13

❓ Frequently Asked Questions

What is the STONE nephrolithometry score?

It is a CT-based scoring system that uses five variables (Stone size, Tract length, Obstruction, Number of stones, Essence/density) to predict percutaneous nephrolithotomy outcomes including stone-free rates and complication risks.

When should PCNL be considered over ESWL or ureteroscopy?

PCNL is generally recommended for stones >2cm, staghorn calculi, lower pole stones >1cm, and stones resistant to ESWL (density >1000 HU). The STONE score helps quantify expected outcomes.

How accurate is the STONE score?

Validation studies show the STONE score has good predictive accuracy (AUC 0.70-0.75) for stone-free status after PCNL. It performs best for stratifying risk categories rather than precise percentage predictions.

What is considered a good STONE score for surgery?

Scores ≤5 indicate favorable anatomy with >90% stone-free rates. Scores 6-8 are intermediate. Scores ≥9 suggest complex cases that may require multiple procedures or access tracts.

What is PCNL?

Percutaneous nephrolithotomy (PCNL) is a minimally invasive surgical procedure to remove kidney stones through a small incision in the back. A nephroscope is passed through a tract into the kidney to fragment and extract stones.

How is stone surface area calculated?

Stone surface area is calculated from CT measurements using the formula: Length × Width × π × 0.25. For irregular or branched stones, the areas of each component should be summed.

Why is stone density important?

Stone density (Hounsfield units) predicts stone hardness and fragmentation difficulty. Low-density stones (<950 HU) are often uric acid and fragment easily. High-density stones (>1100 HU) like calcium oxalate monohydrate are harder.

What complications can occur with PCNL?

Potential complications include bleeding (may require transfusion in 5-10%), infection/sepsis, injury to adjacent organs, urine leak, residual fragments requiring additional procedures. Higher STONE scores correlate with higher complication rates.

📊 Kidney Stone & PCNL by the Numbers

1 in 10
Lifetime incidence
93%
SFR (low complexity)
45%
SFR (staghorn)
5-16
STONE score range

⚠️ Disclaimer: This calculator is for educational and research purposes only. Clinical decisions should always be made in consultation with the treating urology team. Patient-specific factors not captured by the STONE score may significantly affect outcomes. The STONE score provides estimates based on published data and may not reflect outcomes at all institutions.

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